If you were to split workout intervals and sets into two types "long" and "short", HR is pretty much useless for the "short" ones.
I'd say that's anything short of a pursuit (4min) interval. HR ramps in such a way to not be useful.
For longer ones, it could be useful to ensure you're not doing too much work near threshold/suprathreshold when you really want a lot of SS work instead. I, like many, bite off too much for my SS intervals and wind up doing too little time versus planned.
I'd go by RPE and "just do it" probably until I get to those 3x8's and 2x20 kind of things. Then again, since you don't want to stop for a sign or turn or coast for a bit for a roller............that's likely done on the trainer. And if so, you could track your progress by trainer speed and duration treating it like a power meter.
FWIW.........I got a used meter on Ebay for $150. That's a pair of nice tires to some folks.