First off congrats on getting on the bike, building fitness, and improving your health. Nice work.
220 - age has a reasonable sized error to it. Robert Chung will correct, but off the top of my head the standard error of the mean is +/-15 b/min. As an example, i'm 55 next month and my actual max is 190 b/min (if i feel motivated to go that hard).
Anyway, training zones, however they're written or whatever method is used to calculate them are really a way to enable someone (e.g. coach, sport scientist, author, etc) to describe how hard to go in an objective way (rather than, ride moderately hard for 2 hrs, which can cause all sorts of issue)... Polarized is one way to train, there are other models, such as pyramidal or threshold, etc. all models are useful, but some are more useful at specific times. it just depends.
Do you need a power meter to train? I think it tends to make things more effective and is an objective way to measure exactly what you've done (e.g. i rode for 1 hr at 200 W), whereas speed (velocity) is affected by environmental and topographical conditions, HR is affected by multiple factors (e.g., caffeine, traffic anxiety, cadence, temperature, etc) and perceived effort is also affected by external and internal factors. However, it's not necessary that you do need one. You can create a training plan (or buy one, or get a coach - we do both of these) and just use HR or RPE. I even ride my gravel bike with nothing but speed on my bike computer (albeit i am waiting for my power meter for my gravel bike to be delivered).
You've obviously tried a variety of training ideas. that's really good. I'd add in some strength work as well as this will also have health and cycling related performance benefits.
In terms of what i'm doing to stay in shape...i ride about 15 hrs/week, i do 2 to 3 strength training sessions per week. I have a plan. I also look carefully at my energy intake and expenditure (and plan my macronutrients accordingly). I train hard. I still race (season number 41 incoming), my fitness is the same as my mid 20's (actually it's marginally higher), my weight is the same. I do my best to stave off illness and health conditions.