Ummm, you can ride RGT for free, but I've never used the Workout functions in it to know if they're free - guessing you've seen not. I do know to ride from your own .GPX file (made from something or recorded from another ride), you have to import it into a 'Magic Road' and you can only create those with a paid membership. But with a free account you can ride others Magic Roads, so if you can find someone online who's created a bunch (as workouts?) and shared the codes, you should just be able to ride them.
Zwift used to do a free account for a low amount of riding, then one which was distance limited at 100km. Not even sure they do those anymore?!
Other options, Rouvy and Fullgaz. Never used them but they're still going AFAIK.
Most/All of the trainer manufacturors have always had their own 'platforms'. Not sure which ones are free, or if locked down to their own range of trainers. Don't think they're anything more than like TrainerRoad where you don't have a landscape to ride though, just workouts, but that's what you're after anyway. I know Tacx had one with even classic climbs recorded you could ride along to, but think that was paid and then died a death a few years back.
DCR has a list/guide/review bit but it's a couple of years out of date. Although you'd likely find current users in the comments sections. https://www.dcrainmaker.com/2020/03/cycling-indoor-trainer-app-guide.html/
Zwift used to do a free account for a low amount of riding, then one which was distance limited at 100km. Not even sure they do those anymore?!
Other options, Rouvy and Fullgaz. Never used them but they're still going AFAIK.
Most/All of the trainer manufacturors have always had their own 'platforms'. Not sure which ones are free, or if locked down to their own range of trainers. Don't think they're anything more than like TrainerRoad where you don't have a landscape to ride though, just workouts, but that's what you're after anyway. I know Tacx had one with even classic climbs recorded you could ride along to, but think that was paid and then died a death a few years back.
DCR has a list/guide/review bit but it's a couple of years out of date. Although you'd likely find current users in the comments sections. https://www.dcrainmaker.com/2020/03/cycling-indoor-trainer-app-guide.html/