Build plate (PEI on glass) isn't flat after several months of use?
Looking at the picture, the first thing that came to mind was, "are you sure it's the bed?". 

The height variance looks very regular, and while I'm unfamiliar with this printer's specific mechanics, my thought process trended to the Ender 3 and other v-wheel extruder mounts. If the extruder and gantry carriages are mounted to the gantry spars on V-wheels instead of sleeve bushings or other linear guides, and you've spent a lot of time printing small objects where those wheels are going back and forth over a relatively small travel distance for the entire print, you have been unevenly wearing the wheels so they've become eccentric around their rotational center, and this will cause the extruder to vary its height over the glass in a very regular pattern tied to the circumference of these carriage wheels. This happens especially quickly if you over-tighten the bearings against the gantry spars in an attempt to make the printer more precise.

If the printer uses sleeve bushings, the worn sleeve won't rotate, and any wear on the spar will be very localized to the areas in which you print, but if the printer has spent most of its life printing a grid of small objects (tokens, sets of small figurines), you can still get this regular wear pattern as the printer will spend more time over those areas of the bed.

The fix is to replace the wheels or linear bearings if the printer uses them. If it uses sleeve bushings and the gantry spars themselves are worn in this pattern, you might be able to rotate the spars to put an unworn (or less worn) band of metal on the top of the spar, depending on how the spars are mounted into the endcaps of the frame and y-axis carriages.