As others have already pointed out, the key limitation in your system is not the gearing or the cadence on the bike, it is your arbitrary bpm limitation. I'm another 60+-er who would get up no big hills with my current gearing if I had to hold my heart rate at 150. The 220-age formula is considered only an initial guess only as individuals very greatly, and it is now well-known that for more fit senior athletes it doesn't fit well. There are several better formulas for more active athletes, eg.
Maximum Heart Rate = 211–0.64 x Age
Which if you were e.g. 70 would put your max at 166bpm.
Here is an article on this issue which includes this formula. Check with your doc of course. Most docs these days have moved on from the 220-age formula and are aware how all formulae are just an initial approximation.