My strategy for maxing out all characters is the same so this guide is universal.
Each character in the game can purchase 3 Thumps, 3 Rowdiness, 2 Focus, and 2 Imagination upgrades. You will need 2,290 treasure per character (18,320 total) to max them out. You can only carry 999 treasure before heading back up to the hub and any unspent treasure gets wasted (a very aggravating feature). Unfortunately there is no real easy way to grind out treasure but you will find the most treasure in the lower floors or in the Nightosphere. Here is my setup for grinding out those last few upgrades:
Floors: 91-95. By collecting loose treasure, opening chests, and defeating monsters (especially Mimics and Magic Man) as well as generators, you can usually net 900+ treasure in a single run of these floors. If you want to move through a little quicker and not necessarily fight anyone you can still probably get 500+ treasure.
Character: Marceline. She can float over pits and water and, most importantly, can absorb red projectiles (of which there will be plenty in these floors, trust me). Even if you aren't trying to max out Marceline you can still use the treasure you collect with her to upgrade other characters back in the hub.
Sub-weapon: Battle Axe. If you plan on fighting enemies then this is my personal favorite weapon. You can usually find them uncommonly around the later floors and will probably run into a couple while you're grinding in the Gum Hive. It can fell enemies in a couple of hits at most (usually just one hit if used in conjunction with a Wish Orb token) and has a decently wide attack range, but isn't the quickest weapon in the world.
Tokens: Heart Sweater, Speedy Feet, Wish Orb/Wishing Eye, Zeldron's Armor, and Lollipops are all good tokens to use. If you have completed Starchy's final quest then you should have infinite Heart Sweaters which should be sufficient to use when grinding. If you just need a few more upgrades then you could use a combination of Speedy Feet and Lollipops to quickly go through levels, collect treasure, and leave as soon as you have enough. If you're in it to grind through a full five floors and want to get at or as close to 999 treasure as possible then something like Speedy Feet or Zeldron's Armor used with Wishing Eye or Wish Orb (with a strong weapon like the Battle Axe) can give you the edge when fighting enemies to get even more treasure.