@tenantfile I'm sorry to hear you are also dealing with migraines - they are the worst! I'd be happy to share. And I'm sorry this is really long, hopefully there's something useful for you!
My main issue was posture due to sitting at a desk. I was sticking my head out, and shoulders rounded forward. The main things I read about that needed fixing were increasing core strength, stretching out the chest and front of the shoulders, and strengthening the lats. For me, my chest and front shoulders were tight, which pulled my shoulders forward. Then my lats became weak and my traps took up the slack. It got so bad I couldn't do any kind of overhead lift because I would default to using my traps, which would cause tension in them and my neck, causing a migraine. I'm telling you this because it was specific to me, and if it sounds similar to what you're experiencing, then the exercises may help. If your situation sounds different, it might not help so much.
So now, what I did:
M W F:
core work (10-15 minute targeted), and gentle back and shoulder strengthening / stretching (15 -30 min)
T Th:
5x5 strong lifts, slow, very focused on form, especially core and lats.
In between sets, I do some of the arm stretches I learned.
And for a long time I replaced the overhead press with gentler lat strengthening exercises: downward dog, holding plank while pinching shoulder blades together and then pressing through shoulders (I'm not sure of the name of this move, but it's in one of the video names I'll share), dolphin press. I'm now doing overhead press with light dumbbells and getting no headache, so yay!
The M W F work is a bunch of videos I cycle through on the fitness app FitOn. It's free, just has annoying popup ads (no ads in the videos though). I have the videos I mention below favorited and then I just pick two or three to do in the morning. I modified a LOT on the ab workouts.
Abs: the instructor Kenta has a bunch of great ab routines that are easily scalable (for example, early on I kept my head on the ground and just used my legs for many of the movements, because crunching up the shoulders and neck was too intense and would trigger the tension for me). Names of some of his ab videos in no particular order: Crunch Time, OMG Abs, Ab Assassin, Hello Abs, etc - side note, Kenta is so cheery and excited, I enjoy his vibe haha.
Back and shoulder: the instructor Vytas has a couple great ones I cycle through. In order of preference they are titled Shoulder Stretcher, Perfect the Posture, Move Better. He probably has other ones but I've found the stretches and light strength work in the first two to almost directly address my issues gently and effectively.
And that's pretty much it. I will also say one of the best cues I've come across for shoulder posture related stuff is to imagine you are holding a piece of paper in your armpits and squeeze it. This naturally draws my shoulders away from my ears and from rolling forward as well as triggers lats instead of traps.
Sorry again it's so long, I hope it helps!