Part of the reason I do a spinning class is that someone else has planned everything for me although it's easy to go too hard or too easy first few times. A few weeks of doing it and, with some analysis and googling, I can see what the individual sets are (in terms of %-age FTP) and what effect they should be having. The instructor never talks about FTP or similar (as it would be lost on most of the class) but I can now translate the terms he uses for the general gym user into "sweetspot", "threshold", etc. I also have got used to it and know the power that I can aim to hold for a certain period (e.g. a minute, two minutes, etc).
Obviously if you have a turbo at home you can get someone else to do the thinking by using Zwift or TrainerRoad or similar, but all I've got access to is a gym bike that displays power.
As others have said, what you want is variety. Doing 2x20 at 105% FTP is never fun at the best of times and doing it twice a week is a hideous thought, but stuff like 12x1 or 3x8 or pyramids or steps can be much less hideous, and then there's always sweetspot work with sprints and recoveries which rarely feels dreadful.
With just access to a gym bike (and no instructor) I'm using it to do a ramp test[1] every few weeks, and then picking various things from my spinning classes (i.e. 3x8 at
xW or 12x1 at
yW) and try and follow those as best I can in the other weeks.
[EDIT] Here's an example spinning class:
https://connect.garmin.com/modern/activity/4636420949 there's a bit of faff at the beginning with him walking some new people through the "sprint shift" lever and the resistance wheel (they're Stages spinning bikes) but the main stuff starts just after 8:30. If you expand the power graph and then overlay cadence it makes most sense. There are 4 main sections:-
* gradual warm up at 90rpm from easy (100W), medium (160W) and then target as much as you can hold for 1 minute (but make it - I chose 350W), a brief recovery (20s or so) and then straight back into it turning the resistance wheel up until you grind to a halt.
* recovery then 3 sets of sweetspot work with 90s at 90rpm and ~90% FTP and then a minute at 100rpm (without changing resistance) and then a 20s sprint at 120rpm, 50s recovery between and repeat another 2 times
* longer recovery (3 mins) then 9 minutes of sweetspot work of a baseline of 90rpm at ~90% FTP with 3x20s sprints and 1x30s sprint at 120rpm going straight back to 90rpm with no recovery
* 2 min recovery and then similar of 4.5 minutes up near FTP with 4 sprints at 110rpm with no recovery, straight back to 90rpm, one minute recovery (in reality it's longer than this, I just stopped the watch early) and then stop and stretch
Other classes have very different shapes, this one was obviously a sweetspot day, other days often have more of the "all in for one minute" intervals or even a "don't get past 55s" interval where you just keep on piling on the resistance until you're done.
1. With my current FTP of ~210W I do a ~4 min warm up (with a few 5s sprints to 120rpm) and then do a minute at 150W (i.e. start somewhere around 3/5 to 3/4 of your current FTP), then a minute at 175W, etc and upwards all the way until I fail to hold a minute at the next target, then a ~4 min cool down. A reasonable estimate of your FTP is then 75% of your peak 1 minute power. It's only 14-15 minutes for the whole thing and should only be a few minutes of hideousness. The progression is required to build in fatigue, as without the preload I can hold a much higher figure for a minute (I've never managed to complete the 300W minute but, in isolation, I can just about hold 400W for a minute).