In semi-random order :

Farm MR "fast"

Go on the Market. Filter out every weapon that doesnt have a Blueprint.
Craft everything you can. Level up to 30. Sell them if they're not in this list

Do the same for all the labs in the Dojo.

Farm Focus "fast" (why ? because we have to farm 500 fckg to max out a school now.

Sanctuary Onslaught / Elite Sanctuary Onslaught. To do this, see Cephalon Simaris in any relay.

Get Nightmare mods

1 nightmare mission per [time duration] per mastered planet (with the small laurels)
Random mission reward, but look out for Hammer Shot, Shred, Lethal Torrent, Focus Energy, Animal Instinct, Constitution, Fortitude

Get Corrupted mods

By finding Corrupted Vaults on Deimos, and having the correct key in the rapid access menu. Be wary of the debuff !

Looking for Blind Rage, Fleeting Expertise, Critical Delay, Overextended, Transient Fortitude, Vile Acceleration
Best frame seems to be Wukong as he can ignore the debuff of the speed one

Do Arbitrations

Looking for Combat Discipline mod (do NOT upgrade it), and Adaptation (Adaptation is way more important), and maybe Rolling Guard if that's your thing.
Looking for Vitus Essence to spend into the following mods :
Galvanized Aptitude, Galvanized Chamber (at least) => 40 essence
Rolling Guard, Preparation, Sharpshooter eventually. => 70

When the game gives you a Resource Booster (not Resource chance Booster)

Do mining on all planet's open world.

When the game gives you a Credit Booster

If you can, do the Index on Neptune.
The reward is doubled but the cost is not. Be wary though, you should have good weapons as you start without energy.
Start at the lowest level, kill ennemies, pick up the Index Points (most efficient number of points to pick up: 15) and deposit them to a designated bank location (on the map).
Hosting a High Risk Index mission from Clan Dojo navigation will almost always guarantee the Corpus Outpost (ice) tileset.
)

If not, farm Dark Sector missions.

When you need to level up frames, weapons, pets.

Multiplayer :

• Helene, Saturn.
• Hydron, Sedna if you have access to that.

Both because the maps are really small so you benefit most from the Bonus XP from other players's kills.

Equip only one weapon if you want to go fast for it and sell it after : 75% of that XP will go there.

• There's always people for Survival missions as you can farm Resources there.

Solo, for the frame specifically :

• Cetus, Earth. Start the highest bounty possible so the map is full of level 50 or the best level you can reliably one shot. Go on your archwing. Take a high damage projectile weapon. Go up in the sky, shoot, quit the archwing. When the XP comes, go back on your archwing.

When you need to levelup Arch gun / melee / wing

• Salacia, Neptune. I've seen builds with the Amesha wing (drop the two on the objective and keep your one up), and Itzal wing (for the inivisibility, specifically for melee)
• Raijack ?

Farm Railjack Intrinsics

• Railjack. Either the highest difficulty you can join because the range is infinite, or Luckless Expanse (Venus Proxima), which uses your regular Warframe loadout once the main objective is reached. Or stealth killing in Veil Proxima.

Farm standing

Fortuna

Why

To get Gravimag to use the Archguns in normal missions.
For Lenses to unlock and farm new Focus Schools.

Conservation (aka hunting). Go on the small island straight from the doors of Fortuna, equip your hunting rifle, and farm. You can use that on your Archwing for going faster. You can use wukong invisibility to approach the critter without alterting him.

Find the Deck 12 Cave. Mine both parts, and repeat. As the deck itself is a boss area, it is instanced, you can go back and forth ad nauseam.
You need a mining tool, buy the Sunpoint Mining Drill at Smokefinger after a first bounty.

Deimos

Why

To get access to the Helminth, and change one skill to a Warframe from another warframe. (This one is consumed in the operation so you can "delete" warframes you don't like without loosing much)
To get a Panzer Vulpa.

Farm mining as you can sell it to Otak.

Farm bouties. If there is no "Brute Force" mission, start the highest level you feel confident with, go to the mission, then leave and find Mother in the open world. If there is still not this bounty, find another location where mother is (6 or 7)

Cetus

No clues yet ! :p

Quills

Not yet, but anyway build any Amp from Onkko, that'll be better than the default one. Or progress through the story, up to the new war. (don't look up, this is really cool)

Cephalon Simaris

Do Stolen Dreams and The New Strange quests. Then, everyday, do the daily scan for 7k.
Then, if you want more, select the highest bounty on Fortuna, but don't do it. Instead, go to a spot where there is many ennemies, while on your Archwing. Go up so they dont detect you. Scan things. Move a bit so they despawn, come back, rinse and repeat. Don't forget at 50k to buy the upgrade where the scan charges replenish themselves without buying them.

Get pets

Helminth Charger

Does your frame has a strange pink thing on the neck ? If so, you can get a Kubrow egg on Earth, and at incubation, infuse with the Cyst to get a Helminth Charger instead.

Vulpaphyla and Predasite :

Capture weakened on Deimos (Weakened is when they are attacked by ennemies), then go see Son to grow them. You'll need a bunch of rewards from their faction.