You can buy an ant plus dongle for your cranks to measure cadence. They’re pretty cheap but work. That’ll connect to most handlebar speedometers, gps watches, maybe even including your phone if you get the right one.

That’ll allow you to keep an eye on your cadence. You can downshift if you need.

I definitely would not recommend the single speed or fixie route. I’ve ridden those extensively and find myself grinding slow at least as often as spinning fast. They’re fun though.

It’s been probably 20 years, maybe more since I took a spinning class. There was a lot of focus on finding your cadence off the beat of pre-selected music. This literally has stuck with me for decades, and even with no music, I have a remarkably accurate sense of my cadence. It’s my only “musical” talent. Try it, maybe it’ll work.

I (occasionally) use Rouvy for my indoor workouts. On their training side, many of their workouts specifically focus on cadence. I imagine that other brands of workouts do too. It could be worthwhile. Not sure how it knows my cadence without a sensor but it does.

This shouldn’t be too hard for you. You’re already pretty close. It’s not like you came here with 60rpms. And when you get tired, 80 isn’t bad.