Limbo sounds great on paper, but ultimately is incredibly "meh’. The invulnerability is a trap - time to kill in warframe is incredibly quick. Either you survive, or you don’t (and get basically oneshot). Running missions where you would usually die but using a gimmick not to, only results in you going through sub par gameplay, and ultimately leads to less fun with the game.

Have you done void relics? Sounds like you might’ve skipped prime weapons / frames altogether, which would help you a lot, and sometimes is easier and cheaper to get than the “regular” version. I do remember getting ivara prime before the actual ivara, since as you and other people mentioned spy missions suuuuuck.

What you need is a really good frame, a good meele weapon, an ability to move through the game quickly (control + space, aiming where you want to go, is disorienting at first). Try for the ones I mentioned previously, especially Rhino. Frost is a good early game frame, but I think you already are past the point where the bubble is sustainable.

Gauss is incredibly fun for speedrunning a mission - its main skill is “run fast” lol.

Inaros is a “massive health pool” type of frame for armor and survivability

Khora is my favourite, with huge crowd control and absurd damage when built right.

Mesa is a very high DPS glass cannon with some survivability built in. She literally has full auto revolvers that autotarget enemies that can be made to deal an absurd amount of damage.

Trinity is great for supporting other players and for your own survivability. You won’t kill as much as a mesa would, but you will keep everything alive to do the damage.

Titania is great for speedrunning again, she can turn into a miniature archwing that scales with her ability strength, leading her to literally warp through closed doors sometimes.

Wisp is an immortal frame – she can boost speed, vitality and health regen (hence immortality) and she can stun everything around by simply standing with a buff up. She’s great for defense (some objectives can take her buffs and heal too) and her ultimate can carry you quite nicely till you get too far in enemy levels, at which point it kinda drops off in damage.

Were I you, I’d go for wisp if you have the boss unlocked already. Just go with randoms and quite often than not, there’ll be someone who is able to deal incredible damage to it. For a meele weapon - I had a reaper prime with lots of mods that made it deadly. It sweeps in large arcs, does plenty of damage, etc. No idea if it is possible to get it now or how viable it is.

Dual ichor is a weapon you can get easily by joining a developed clan - they’re cheap and can be built into a literal blender. They’ll serve you well till you get something better