Replacing/adjusting the heated segments to make it coplanar?
I've noticed there's quite a lot of variance in height between the heatbed segments, which leads to a somewhat wobbly surface when the print surface is attached. Bed leveling ensures a nice first layer, but this will still leave a not-flat surface on a larger print. It looks like there might be some leveling screws as part of the bed assembly, but I thought I should ask here before just randomly tightening/loosening thing.
The heatbed tile is for sale, but the store page just links to the general documentation site for information, and searching there doesn't turn up any results for how to install/adjust them.
Light gap visible between a straight edge and heatbed segments.
Add silicone washers?
I was wondering about that as well. I remember the HUGE improvement that it made on my mk3s when I did the schweinert silicone bed level mod. Not using octoprint with my XL, my plan was to attach an indicator to the toolhead (starting with printed dial indicator and xl-toolchanger-indicator-holder) to get the tile corners all to the same height.
There's a ~5mm raft of foam between the frame and the tiles. If that's got enough flex, the fix should be pretty straightforward, otherwise, it'll be time to break out the spacers.