There are certain weapons I consider platinum only, specifically because they are locked behind a piece of content that has too long of a lock for me to keep my eyes open.
Specifically, I'm referring to things like the Arum Spinosa, Sporothrix, and Deimos Archguns, as you need to run Arcana Vaults, which might as well be solo only, and annoying/boring as well. The Ambassador and some Prime warframe (Ash?) parts are behind 20 min Railjack Survival. Vandal Lato/Braton are at the end of 20 min ESO. In addition to being mind numbing, it's also very easy to fail/get lesser rewards from these missions.
I would generally only subject yourself to a grind if the secondary/tertiary rewards are worth something to you. Running relics is almost always worth your time, regardless of mission type. If you need to level some MR fodder, run some Void Cascade or Duviri Circuit so you're also getting Zariman Arcanes or Circuit rewards, as well as big bunches of Focus.
Personally, those 3 farms are better run separately.
Simaris standing I use Ivara on the Exterminate mission on the Kuva Fortress (Steel Path if you can). Just stealth through the level scanning enemies and sentry guns, not really killing enemies as you go. Exterminate missions have a mechanic where enemies will start pathing to the exit if you bypass too many of them, so just follow the crowds to the exit. Then just use a melee weapon to clean up the crowds at the exit after scanning them.
Focus farming, do Duviri Circuit (Steel Path if you can). The Thrax enemies that spawn constantly are worth boat loads of focus. You can also do Void Cascade, deep in the game, but it's much less accessible and harder, having full access to your armory might offset the downsides.
Kuva weapons, just do the capture mission on Saturn. Easy 2 min sprint. Also, Corrosive Kuva weapons? You can only get Radiation or Magnetic dual-element bonuses.