All of my early gains (2014-2015) were from doing Tue/Thur spin classes plus an easy outside ride or two. Based on 4 months off a couple years ago, guessing that my ftp went from ~180 up to ~250W simply riding roughly 2 hours in the gym and averaging 4 hours/week. Those gym sessions had some very hard, high-intensity intervals.
Then I bought a road bike at start of 2016 and over 7 months ramped up riding outside to average 8 hours/week and FTP jumped to ~280. Scaled back riding a bit, then in 2017 on 7.7 hours/week average and more focused hard sessions, FTP returned to ~280. Did a lot of really hard workouts, two a days, and longer rides.
After that I bought a trainer, reducing riding to TR MV levels, about 5.5 hours/week, and FTP saw yo-yo between 220-250.
Then in 2020 I switched back to a volume approach, increased riding to 7.5 hours/week with a lot of easy rides, and saw FTP jump 260. Continued down that path this year (2021), with a minor 15 min/week increase and FTP jumped to 264W. Mostly easy rides and starting to approach where I was in 2017, and setting new power PRs again.
Volume (7.5+ hours/week) with some intensity has been better than indoor structure, for me. I’m closing out my fifties and fighting age related declines, I’m pretty happy with averaging 7-8 hours/week as a stress reliever and the Watts that come from that level of volume.
Volume, consistency, and a multi-year horizon are generally acknowledged as the fundamentals to success in endurance sports.
Hope that perspective helps.