I used to work a bit in rehab and with back problems, depending on what the issue was it is often related to core strength.
I like to swim so I go to the pool a couple of times a week. I do some aqua aerobics type classes but there are loads of people who walk laps in the water. Really low impact but high resistance!

That's what I have seen also. Anytime I work to build core muscles, I strain my neck, causing headaches, therefore the reluctance to try again! Any neck building exercise recommendations (that are gentle) would be appreciated. I've done PT for this, but I feel since there's a time crunch (wonderful insurance limiting number of visits) they add the core, and arms in my case, too soon! Which leads to an aversion to all.

Maybe try just the neck, and build until ready, then add arms, or core (one at a time), hopefully leading to tolerance of all! (This is really advice to myself, but can help others too). I'm open to ideas too!

I've always been info lifting and not overly fond of cardio but sadly I'm coming to terms with the fact that I can no longer lift due to RA. It causes too much joint pain and triggers a flare. Plus any activity where my arms are raised triggers muscle spasms in my neck. So for now I walk and hike. I just started doing the online Barre3 workouts which are kind of a blend of yoga, ballet, and Pilates. So far they seem to be kind to my joints so I'm going to focus on those with hiking when weather permits.

Yes, I was scared!! Contact your doc and ask for a PT referral. I worked with PT until I felt stronger & then set up some sessions with a trainer at a gym after that. (I had back surgery last year after 20 years of issues). My trainer helps to make sure I'm using proper form, so I don't hurt myself & shows me how to modify the things I should be modifying. After about 10 sessions, I now have the strength & knowledge to start group classes, which I recently did. I'm always going to have to modify some things, but at least now I know!