Need advice for an end grain board

https://imgur.com/a/jc06AO5

The link will show my issue. I’ve all but got the final touches on this commissioned board. It is 23.5”x16.75”x1.375”. With it being too wide for my planer, and not having a drum sander, I used a router and sled to flatten it, which worked worked perfectly. Life being what it is these days, once I got it flat and sanded, I left it in my garage until I could get back to it a few days later. When I came back to it, it had a bow of about 1/4” in it. I figured it was due to the colder temps we’d had (was about 60° the night I flattened it), but I’d also done all my milling, cutting, and gluing when it was 40-50°.

So I took it inside, and the bow went away. Well, I still haven’t gotten back to it and the bow is now back, though not as pronounced this time. I’ve noticed it fluctuate just being indoors. I’m located in a very dry climate, but temperatures can range from 20-70 in the winter. My house stays pretty much between 71-72°.

My question is, do I need to get it flattened, sanded, edges routed, and oiled (Walrus Oil) all in one session? Should I just keep it indoors until the movement stops? Any advice would be greatly appreciated, as this is my first commissioned piece. Thanks!