There's a cheeky exploit you can do by trading stuff with a caravan that's right outside of a town. Took me like 15 minutes or so to go from ~125 leveled up naturally to 300. Preparation time to get the required materials took me about 10 minutes or so.
It is very upsetting though, because leveling up trading by itself barely helps you make any more profit than you normally get, the difference between 100 and 300 feels negligible. Profits are very slow, so unless you're supplementing your denar generation with smithing or at the very least farming lords, you'll barely have enough money to purchase towns.
I've made 3 characters so far since I started playing Bannerlord, and wanted to try being rich by trading. It was slow and boring but much more profitable than being a mercenary or a vassal, considering I didn't need to pay upkeep for a large, high tier army. Then I found out about the trading exploit, and did it all the way to 300 to see if things were going to get any better. They didn't, profits were largely the same. Some of the perks are nice, like spring of gold which helps cover expenses really nicely for troops and food, but that's about it.
I wanted to buy a town, and it cost about 6 million denars. I increased the relations with the lord, and managed to drop the price to like 4 million denars or something, I don't remember the exact value. After 2 in game years of mostly trading while still trying to do other things like tournaments and some quests or whatever because I wanted to have fun as well, I had accrued a whooping total of 1.4 million denars...
I had 3 workshops producing about 300-400 denars each, one caravan that was producing in between 200 and 500 daily, spring of gold, and a fairly small upkeep of about 400 denars. So daily I was getting about 1700-2300 denars daily or so, and about 15-20k on each run selling desert/aserai horses.
So I decided it was just going to take a very long, tedious and boring grind to hit the 4 millions denars I needed to buy a town, gave up and went level smithing. Took me about 20-30 minutes to hit about level 100 and I was already crafting 2h swords that went for sale at about 25k-30k denars each.
Yup.
Leveling up trading is a joke, and if you want to get any use out of the level 300 perk you need other ways to actually make denars like smithing or lord farming. The only good things leveling trading provides is getting renown from your workshops, more carrying capacity (can't remember if those are trading or steward), and spring of gold for a fairly stable source of daily income if you already have some denars saved up without the hassle of taking care of fiefs and their garrison.
Sorry for the long and slightly unrelated rant, but trading in general needs an overhaul.
Firstly the benefits you get for leveling trading up are close to meaningless for your profits. As such, most perks that are of the "decreases your trading penalty for XYZ good", are also meaningless and barely have any effect on your profits.
Secondly, there's very few perks that are actually good. Renown from workshops, carrying capacity, spring of gold and buying settlements are the only good ones, or at least, that have any meaningful impact in the game.
Thirdly, the perks that do things like decreasing upkeep of workshops/increasing workshop production seem like a better direction for trading perks in general, in my opinion, but since workshops themselves are already not very reliable, you don't get a lot of value out of them. At least, it's very hard to percieve if they are actually having any impact on your workshops.
TL;DR
Just do the cheeky exploit to level up trade IF and ONLY IF you are looking to get the perks for renown from your workshops or spring of gold. If you're planning to level up trading to increase your profits and use it as your main form of income, save yourself the pain and don't waste any focus points on it. Too little impact on your profits for it to be worthwhile whatsoever.
