Combining Elevation and Bathymetry Descete Color Scales when 0m sea level varies

[rhgof]

I am trying to plot elevation multiple elevation raster that has +/- values representing ocean depths below sealevel and elevation.
I want to create multiple plots with 0m-level changes reflecting impact of sea level changes.

My challenge is getting the 0m sea level to align with the "zero" color on say gmt_global palette.
I can get it to work with continuous fill scale by setting

 scale_fill_hypso_c(palette = "gmt_globe",  
                          limits = c(-rastMax, rastMax),
                          )

But want to do it this with discrete scale.
As the zero datum changes with each plot it looks like I have create a manual color scale every time to make this work which would require calculate # of levels below and above zero-datum and then combine two different color scales to make this work reflecting different number of levels of below and above sea level each plot.

If raster goes from -1000m to +1000m and I have 50 bins to plot this as change sea level by 40m per plot I have to change the scale from (25,25) colors (below, above) to (24,26), (23,27) with each change in 0 datum

And to make this harder I want different breaks above and below sea-level reflecting different land gradients in flatter areas near the coast (above and below current sea level)

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Update: I think the answer is in here - and some remapping of the limits for the scale I want to use.

Hypsometric Tints DB