How could a Ryzen 5 series hold a GTX 1070 ?
The framerate is stable at 65 FPS with very rare drops to 64. But I was on field close to a savannah and desert so there weren't much things that could drop the performance tbh.
Having 60 FPS is perfect for me but I know that once I install RTG (Realistic Terrain Generation) and Biome O' Plenty + plenty of secondaries mods, i'll get a 20 FPS drop (worse I had was 26 or 27 FPS)
I would just try it with your mods on a locked framerate (like 65). I had similar issues like a year ago, my performance in minecraft was all over the place if I didn't lock the framerate. Not quite sure how I fixed that, reinstalled minecraft and used the standalone launcher which requires you to install java yourself, might helped with it.