I’m glad you were able to finish the meet and get a couple PRs in! Are you heading right back to work, or will you take a day off for your back?
For the activation stuff I really like to start laying on my back, feet on the wall, femur straight up, shins parallel to the floor (90/90). At first you can just try to relax and let hip flexors/midle back and groin unclench.
From there kinda squeeze your hamstrings like a leg curl and flex your lower abs like a McGill curl up (but keep your feet in the same spot). Hopefully you can hold this position and let your lower back un-kink and relax. And then squeeze more abs and hamstrings.
After that you can start trying to get your arms over your head (on to the floor). If you’re too tight go 1 arm at a time first. If you can get your traps and deep hip flexors to chill out you should be able to get scapulas loose your lats working.
After a few minutes working/breathing in the 90/90 position you can squeeze a pillow or foam roller between your knees to see what’s up with your groin and adductors. Sometimes mine really feel like shit and I almost get a cramp from this. If your SI is jacked up or crooked sometimes this is enough to “reset” it. It feels real good if that happens.
If all of this stuff is too easy you’ve got clamshells, dead bugs and glute bridge/pelvic tilt moves as almost "opposites"of McGill 3. But sometimes it’s hard to move around much with your back tight so I like to start easy.