Advice about my new mattress/topper/back pain

52M, 180lb side sleeper, just replaced our 10yr old Saatva with Sealy Posturepedic Pro Lincoln Heights 14" Firm from Costco, after exchanging a couple others that were too soft, about 3 months ago.
I have general mild lower back pain that honestly I chalk up to age, and the fact that carrying a guitar around my neck for hours on end is my job. With the aging Saatva, I would have mild back pain all day, same with The Leesa Legend we initially got at costco a couple months ago.

So I figured it was time for a firm mattress. The Lincoln Heights was unsleepable for me the first night, it was so firm. we had to makeshift a topper just to get through the night.
The next morning, I ordered a cheap combo topper memory foam thing from amazon. 2" MF and then the quilted topper thing($100). It was perfect for about 2 weeks, and super comfy/cloudy.

Fast forward 10 weeks to today, and I can get about 5-6 hours of great sleep, then my lower back and hips start to hurt, and I toss and turn.
Plus, the cheap memory foam definitely gives me the feeling of being "stuck" in it, which I'm not a huge fan of.
I will say that I fall asleep immediately and stay that way for 5-6 hours.

What's interesting is that after I get up, and "stretch it out", the pain subsides in an hour, and my overall level of back pain and general back "health" seems to be better throughout the day, than it was before! I don't get achy in the late afternoon or after doing guitar stuff. I attribute this to the firm mattress?
So I feel like I'm really close to reaching mattress nirvana. I don't mind spending money to get this right

Questions :
Should I be looking at another kind of topper and or memory foam?
Does the super firm bed just need more break in time?(it's been 3mo)
Is the Mattress too firm and I need to start over? (please god no)
Do we need to re-evaluate the foundation ? (slats, 3" apart)
Do I need to learn to sleep on my back?

Thank you for your insight!