RPE or HR based TR outdoor workouts

I typically train with power almost 100% of the time, I use the same power meter on all my bikes. I’m in a temporary situation where I only have power on my road bike which is on the trainer. It looks like I’m going to have some free time due to the worlds craziness right now and I’m definitely riding outside since I have a new gravel bike to explore on. I’m also going to get the mountain bike on the trails again. Neither bike currently has a power meter yet.

Still, I want to maintain as much compliance with my plan as possible. I’ll have to nail my key workouts indoors but I think I’ll use the opportunity to gently raise my overall TSS with some endurance or SS work.

I’ve been so married to power now I’m a bit concerned about trying to nail a good workout on the bike without it. Does anyone have any advice? I’ll use a HR monitor to help estimate TSS, but I figure RPE is more effective for any intervals.

Would you push endurance or SS workouts to a head unit and do the prescribed workout or just try to spend time in that zone(s) riding free and enjoy the time on bike? I fear over unders, threshold, or VO2 Max is probably best to leave indoors.

Thanks for your thoughts!