Let’s remember why many of us ended up here in the first place — to get faster!
Structured training is an extremely effective way to increase your fitness over time. While many of us are itching to get outside after a long winter, there isn’t really a good reason to leave structured training behind if getting faster is your main goal.
It’s normal to want to ride your bike with no strings attached purely for fun when the weather is good (that’s probably what got us all here in the first place), and we advocate doing whatever it takes to stay mentally motivated to keep riding your bike. There are ways to make room in your training plan to allow these types of rides to happen regularly if you’d like, such as bumping down the volume of your plan to give some breathing room for soul rides such as these.
Riding with a power meter is going to be beneficial if you’re really serious about your training, but @NateP is right — completing your workouts based on RPE is totally acceptable as well.
As @Stevie_Dee mentioned, there are benefits to having TR around aside from simply riding on a trainer or even having workouts at your disposal. Having access to your calendar, FTP Detection, Adaptive Training, and analysis of your progress throughout the season is a huge asset, especially when training for something such as the Wales Ironman!