A powermeter definitely isn’t essential, you can use RPE (Relative Perceived Exertion, basically “how hard does this feel” on a scale of 1-10) to get a pretty effective structured workout, particularly when you’re starting out.
As above, don’t worry too much about cadence, definitely don’t need to keep it over 90 all the time, though 70 is towards the low end of what’s normal unless you’re climbing.
I would say that you need to be realistic about balancing both. They’re pretty opposite ends of the scale in terms of the ideal physique, unless you’re talking about very short, explosive cycling events like track sprinting where they carry a lot of upper body muscle. No reason you can’t be pretty ripped and also decent on a bike, but at some point the training for each is giving conflicting messages to your body and you’re going to hit limits as to how much muscle you can build if you’re out doing 2+ hour bike rides, or how big an aerobic engine you can build if your muscles are shredded from pumping iron every other day.
My advice would be to get yourself a code for a TR free trial and try some structured outdoor workouts using RPE. Start out with a sweetspot plan which basically involves longish intervals at an intensity that is hard but not soul-crushing.