You might ask yourself "how much do I need to get all the upgrades?" and the answer is...

53500 Gold

That is a big number, though 38500 of that is because of Key's shop, which if you're going for all the upgrades requires 3000 gold before you get access to them, then 11000 to get all of them. If you're just going for the mushrooms from his shop then you need to drop an exorbitant 24500 gold to get them all.

The remaining 15000 is spread between all the things Milk related and the cash requirement to unlock weight level 4.

So how do you get all that dosh? The best method requires fighting the bonus bosses and taking their loot, but they're late in the game so at that point you might already have scrounged up most of the funds needed.

The alternative, however, involves a certain pink dragon and their flying course.

You'll need to get to Slime Caves 9 for this, as that's where Kitora's course is. If you've beaten the Slime King at this point and opened the shortcut, you can easily just fly to Piglands 9 and then go down to the left until you find Kitora.

If you somehow haven't done the flying courses yet, you might want to do the one that's at the entrance to the mountain route right next to the bird statue in Piglands 4, since it will teach you on how play the minigame (and isn't a bad place to grind before you get access to it, but it'll take twice as long to grind than doing it here).

This stage is a bit tougher than the previous one, but can net you at best 700, or at least around 600 if you're diligent at grabbing coins. In fact, I recorded a vid of the best route one can take for this stage:
https://vimeo.com/678359677

The vid itself shows that the run is about 2 minutes long in total. At an average payout of about 600, you'll have to do this for...
*plugs numbers into calculator*
about 3 hours or less, but that's not factoring in failed runs (which can give you some profit, but most likely less than 500 or so) and any gold you had before doing this grind.

Not piss away 3+ hours of your life making an inflating plush grab coins?

In all seriousness you can just grind to get the speed belt (which costs 5000), but that's still like 20 minutes or so of grinding.

The only other way this early you can do is fight the crystal enemies in the crystal caverns since those guys drop a lot of gold upon being shattered.

And if you're wondering, No, Kitora's course in the Crystal caverns is not worth it as the payout is similar to doing the slime course perfectly, coupled with the fact that it's hard as balls... unless of course someone can figure out a fool-proof route for it. (or the devs nerf the difficulty of it)

(I neglected to mention the treasure chests that contain the platinum coins that are scattered about but those don't give much either or, again, are too far in the game to be much use)

And there you have it. Up to you if you really want to follow this guide or not, I can't force you. If any later updates add more upgrades that require gold I'll update this guide when I get the chance to, but if not, fare thee well.