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I am not entirely sure if I understand your question correctly. Ther e, I restate my interpretation of your problem before proposing a potential solution below.

I am not entirely sure if I understand your question correctly. Therefore, I restate my interpretation of your problem before proposing a potential solution below.

EDIT: Added a potentially improved version below.

If this is the issue you are trying to resolve, the following function could address your needs. Additionally, you can map the function to szz. Note, that this solution does not work well with lines that do not fit the entire screen.

Following up on @TheGiwi comment, I came up with the following simpler solution. I believe the solution to be more stable. However, it still struggles with soft wrapped lines that cross the screen boundaries.

EDIT: I added a second version below, which is simpler but leads to more errors for the author of the question.

If this is the issue you are trying to resolve, the following function could address your needs. Additionally, you can map the function to szz. Note, that this solution does not work well with long lines that are soft wrapped around the screen boundaries.

Following up on @TheGiwi's comment, I have came up with the following simpler solution. I believed the solution to be more stable, but it appears I was incorrect. It seems that, for @TheGiwi, the first solution works better. Like the initial solution, this one also encounters issues when long lines are soft wrapped lines around screen boundaries, resulting in errors.

I am not entirely sure if I understand your question correctly. Therefore, I restate my interpretation of your problem before proposing a potential solution below.

function! SoftLineWrapAwareVerticalCenterToMidScreen ()
    " Centers the current line vertically on the window. Soft lines
    " are centered such that their middle is at the middle of the
    " window.
    execute "normal zz"

        let l:nchars = strlen(getline("."))
    let l:width = winwidth(0)
    if l:nchars <= l:width
        " There is no need to further adjust the centering
        return
    endif
    
    " The line is longer than the window width, so we need to
    " adjust the centering. We calculate the number of soft lines
    " of the current line based on the number of characters in the
    " line and the window width.
    let l:nsoftlines = nchars / l:width

    " In order to adjust the centering, first we need to calculate
    " what the soft line is to which the window was centered. 
    let l:midSoftline = l:nsoftlines / 2
    let l:midSoftline = l:midSoftline + 1

    " Now we need to calculate the soft line at which the cursor is.
    " This is the line with respect to which we want to center the 
    " window.
    let l:cursorPosV = col(".")
    let l:cursorSoftline = col(".") / l:width
    let l:cursorSoftline = l:cursorSoftline + 1

    " The difference between the soft line at which the cursor is
    " and the middle soft line is the number of soft lines we need
    " to move the window in order to center the cursor.
    let l:softlineDiff = l:midSoftline - l:cursorSoftline


    " If the difference is positive, then the cursor is above the 
    " middle of the window, so we need to move the window up.
    " Otherwise, we need to move the window down. 
    if l:softlineDiff > 0
        while l:softlineDiff > 0
            " Simulate pressing <C-y> to move the window up
            call feedkeys("\<C-y>")
            let l:softlineDiff = l:softlineDiff - 1
        endwhile
    else
        let l:softlineDiff = -l:softlineDiff
        while l:softlineDiff > 0
            " Simulate pressing <C-e> to move the window
            " down
            call feedkeys("\<C-e>")
            let l:softlineDiff = l:softlineDiff - 1
        endwhile
    endif
endfunction

map szz :call SoftLineWrapAwareVerticalCenterToMidScreen()<CR>

EDIT: Added a potentially improved version below.

I am not entirely sure if I understand your question correctly. Ther e, I restate my interpretation of your problem before proposing a potential solution below.

function! SoftLineWrapAwareVerticalCenterToMidScreen ()
    " Centers the current line vertically on the window. Soft lines
    " are centered such that their middle is at the middle of the
    " window.
    execute "normal zz"

        let l:nchars = strlen(getline("."))
    let l:width = winwidth(0)
    if l:nchars <= l:width
        " There is no need to further adjust the centering
        return
    endif
    
    " The line is longer than the window width, so we need to
    " adjust the centering. We calculate the number of soft lines
    " of the current line based on the number of characters in the
    " line and the window width.
    let l:nsoftlines = nchars / l:width

    " In order to adjust the centering, first we need to calculate
    " what the soft line is to which the window was centered. 
    let l:midSoftline = l:nsoftlines / 2
    let l:midSoftline = l:midSoftline + 1

    " Now we need to calculate the soft line at which the cursor is.
    " This is the line with respect to which we want to center the 
    " window.
    let l:cursorPosV = col(".")
    let l:cursorSoftline = col(".") / l:width
    let l:cursorSoftline = l:cursorSoftline + 1

    " The difference between the soft line at which the cursor is
    " and the middle soft line is the number of soft lines we need
    " to move the window in order to center the cursor.
    let l:softlineDiff = l:midSoftline - l:cursorSoftline


    " If the difference is positive, then the cursor is above the 
    " middle of the window, so we need to move the window up.
    " Otherwise, we need to move the window down. 
    if l:softlineDiff > 0
        while l:softlineDiff > 0
            " Simulate pressing <C-y> to move the window up
            call feedkeys("\<C-y>")
            let l:softlineDiff = l:softlineDiff - 1
        endwhile
    else
        let l:softlineDiff = -l:softlineDiff
        while l:softlineDiff > 0
            " Simulate pressing <C-e> to move the window
            " down
            call feedkeys("\<C-e>")
            let l:softlineDiff = l:softlineDiff - 1
        endwhile
    endif
endfunction

map szz :call SoftLineWrapAwareVerticalCenterToMidScreen()<CR>

Following up on @TheGiwi comment, I came up with the following simpler solution. I believe the solution to be more stable. However, it still struggles with soft wrapped lines that cross the screen boundaries.

function! SoftlineWrapAwareVerticalCenterToMidScreen2 ()
    " Get number of lines of the current window:
    let l:winHeight = winheight(0)

    " Get the line number of the cursor in the current window:
    let l:cursorLine = winline()

    " Middle line of the window:
    let l:midLine = l:winHeight / 2

    " Difference between the middle line of the window and the line:
    let l:lineDiff = l:midLine - l:cursorLine


    " If the difference is positive, then the cursor is above the
    " middle of the window, so we need to move the window up.
    " Otherwise, we need to move the window down.
    if l:lineDiff > 0
        while l:lineDiff > 0
            " Simulate pressing <C-y> to move the window up
            call feedkeys("\<C-y>")
            let l:lineDiff = l:lineDiff - 1
        endwhile
    else
        let l:lineDiff = -l:lineDiff
        while l:lineDiff > 0
            " Simulate pressing <C-e> to move the window down
            call feedkeys("\<C-e>")
            let l:lineDiff = l:lineDiff - 1
        endwhile
    endif
endfunction

" Added <leader> because I personally did not want to remap my `j` and `k` keys at any time. 
map <leader>j gjszz
map <leader>k gkszz
``