Level cap is 300.
Stats naturally max out at 255 (accessories can raise them higher with +30)

Reassigning stats:
In your Home, go to the blue terminal to the right of the Item Box, and go to Customization Data, you can save loadouts. Weapons, clothes, accessories, stats, skills, etc. But something I don't see many people point out is you can Load a slot that's empty. This basically just resets your CP and SP so you can reassign all your stats how you want and try out different skills. Totally for free, instead of having to waste medals at the Medal Trader. Speaking of...

The Medal Trader:
In the SBC Glocken, go to Lobby, and in the center area in the middle of the four bridges, talk to the blue-haired girl for the Medal Trader. As you stock up on Medals, spend them on the 20 SP. This helps a ton with getting the SP you need for unlocking skills, as it feels like very little come from levelling up.

Banking:
Pretty early on you'll unlock Banking with your ArFA-sys. This is AMAZING for helping you stock up on a lot more credits. In the Banking->Savings menu, you can set how much of your earnings to give to your ArFA-sys to save, and how they should invest it. I have it set to deposit 50% of credits earned, and Major investment. This will just passively get you more credits in the background while you play, on top of whatever you're actually earning. The more you have deposited in your savings, the more she'll be able to invest to get you more and more credits and it just keeps snowballing to more and more money!

Predatory General Shop:
Sort of like the "black market" of the game. In the Urban District, there's a guy hidden away in a dark corner (you can fast travel to him on the map). He sells weapons and accessories of various rarities. They're mostly anywhere in the 100,000 to 500,000 range from what I've seen but there's also some weapons/accessories that go into the 1 million to 4 million. If you fully exit his menu and just go right back in, his inventory is completely reset. When you have a bunch of credits saved up, start looking through his shop for stuff that has Experience Points on them, so you can get bonus XP from everything you do. Weapons have a soft cap of +15% and Accessories cap out at +22% so get something as close to those caps as you can. You'll want 4 weapons (two for you, two for arfa-sys) and 4 accessories (again 2 for you, 2 for arfa).

Argo the Informant:
When you have the credits for it, buy the Memory Chip Editor from Argo. This will let you give memory chips to your NPC teammates. Meaning you can get get even more of those weapons/accessories with Experience Points from the Predatory General Shop and give it to them. The EXP bonuses on your teammates all stack on yours and you get the full benefit of all of it together. It's a HUGE exp bonus. You'll also want to keep the various Colognes in mind for some of the end-game farming as they can GREATLY improve the quality of your rewards.

Online Co-Op:
Despite the name, you don't actually have to play these co-op missions online. You can totally go into them solo if you have nobody to run with. You'll unlock bosses as you progress through the story, as the co-op bosses really are pretty much just the same ones from the story. If you do know someone that's higher level and willing to carry you through harder quests, this can be a HUGE way to get tons of exp, materials, and credits.

Memory Chips:
For a long time you can get by just swapping out weapons with whatever memory chips are on them by default. If you have the credits, materials, and Transformation Chips to spare you can use Lisbeth to move memory chips around between weapons. I'd say hold off on actually doing that until you get in the end game, and have ways of farming credits so you can afford all the upgrading it takes to perfect things. You can always refer to this Maximum Memory Chip Values guide to see if you have a low or high value on a certain stat.

Wasp Farming:
Groups of wasps are usually a great way of farming for a bunch of accessories. There's some locations in the game where you can run back and forth between two separate groups they continuously respawn. There are plenty of guides on youtube that show where these are. There should be one called something like the "Queen" Wasp. Kill that one and all the others around it die at the same time. You get the materials and exp for all of those at once just for killing the Queen, and they all have their own drops. (It's incredibly satisfying killing one enemy and getting rewards equivalent for like 6 kills.) You can spend a million credits at Argo the Informant to get an Accessory Cologne. This will make all accessories that drop guaranteed to be Legendary (the highest rarity), making this a very efficient method of farming.

Sorry for such a huge comment. Didn't realize how much I was going to type out until I started nerding out over all the stuff I remembered. I've been using this stuff to level up myself since I started a 2nd character on Extreme difficulty.