Two further suggestions that help me:
Get a backlight for your monitors (cheap amazon things that plug into USB). This decreases the contrast between the bright monitor, and the darker background.
I also wear anti-glare glasses.

As a graphic artist of 30+ years and fellow migraine sufferer, good monitors are worth every penny for me. Dealing with a migraine today for that matter.

I use an LG Ultrafine 5K 27” in combination with my 13” MacBook as the secondary screen and this setup has been really good for me. I’ve had larger screens, which are always tempting, but find 27” is the sweet spot. Larger screen means more light and not quite as sharp. Which brings me to the 5K resolution. 4K is good but the 5K really helps me with eye strain and it may not seem like a big deal but it’s made a big difference for me.