any good guides for levels 50+?

Hi Tenno.

I've been away from the game for a long time, but Warframe 1999 looked so cool I decided to come back. I've blitzed all the story content (holy crap The New War was mind blowing) and finished up the quest for 1999. I've also been going back and clearing much of the map, but I can't quite figure out how to get powerful enough to deal with the higher levels. I'm already potatoing my weapons and frames (do people still call that potatoing?) and using Forma to increase my mod capacity. I'm throwing in as much damage as I can, and trying to match damage to the enemy type, but I just can't seem to keep up with the enemy health/defence at higher levels. And there are so many systems now that its a bit overwhelming to figure out for myself what to focus on...

I'm MR 15. Any mod tutorials I find are a little too early game/beginner-focused, or just so old that I worry they might be out of date, so is there any good up to date guide on how to deal with power progression around this stage? Or just straight up advice from you Tenno would also be appreciated :)

extra info: The mods I use are all either max level, or close, and I'm still in the process of getting that last level or two. My issue isn't with acquiring mods frames or guns, its in deciding what to use, and how to decide what mods to use to make a good effective high level build that actually kills stuff at high levels.

Update: Just wanted to say thanks for all the great advice. I've started to diversify my damage avenues and focus on the weapon's strengths. So if it has a natural crit chance, go all in on that, got myself a nice crit focused Soma prime with crazy fire rate that melts, and a status focused melee weapon that chews through hordes. I've been using the 1999 eliminations as a test and I've gone from taking forever, barely making it out alive to a complete breeze. I'm a lot more confident opening up the map and doing higher level missions now, so I feel like I can start making a real attempt at focusing on all the other systems now (arcanes, riven mods etc)