As has been said, D&D treasure is tied to the game world--so a "+5% gold looted" mechanic isn't something that exists. Either the coins are there, or they aren't.
That said, there are a number of magic items in D&D 5E that create things of value at little to no cost. These can be leveraged by a traveling character to make money from nothing but the magic item they have acquired. This, of course, always assumes you can find a market for what stuff you've created.
This magic item can, once per day, produce certain amounts of liquids. For the sake of legality and maximum profit, I recommend using it to produce acid. The jug produces 8oz of acid. A vial holds 4oz, and a vial of acid is worth 25gp. So, your alchemy jug can conjure 48gp worth of acid daily (a vial costs 1gp).
Thrice per day, this cauldron can turn 30 gallons of water into a hearty stew sufficient to feed 120 people. This stew could be presumed to count as, at least, a 'Modest' meal which, according to the Food, Drink, and Lodging table in the PHB is worth 3 silver pieces. Theoretically, assuming you can sell out your stew, this means that every day you can convert 90 gallons of water (basically free, assuming access to a body of water or good well) into 36gp.
Not hugely profitable in most situations, but can ensure you always have water for your Cauldron of Plenty. This could turn properly profitable if you're in a desert or other locations where water is scarce and you can sell.
These are, alas, consumable--but a single pot of this allows you to cover 1,000 square feet of a surface, and you may 'paint' objects worth up to 25gp that are permanently created. And, importantly, you don't have to be a good artist to use them--they magically flow into the form you are concentrating on.
So, I suggest finding a gemstone worth 25gp and 'painting' copies of it. Or, perhaps, 25gp art objects. It'd be terribly hard to figure out exactly how much money you could make in this way, because it depends on how big the thing you're painting is...but this 1,000 square feet of 25gp gems would likely be a large number of gems. Or if you wanted to go straight to gold...
Just to use Waterdeep as an example, Waterdeep: Dragon Heist tells us that a copper Nib is about the size of a thumbnail, and a gold dragon is about 50% bigger than a Nib. According to this, we can estimate that a typical human thumbnail is about 0.35 square inches--so we can estimate that a Gold Dragon is about half a square inch. 1 Square foot is 144 square inches or 288 gold dragons, so we can ballpark that one pot of paint could produce something like 288,000 actual gold pieces.
Caution: this is certainly counterfeiting...but gold coins are worth their own weight, so just making little squares of gold the size of a gold dragon would be legal.
(Wow, I went deeper on that rabbit hole than I meant to)
Non-magic item means
As is laid out in this answer, gambling is a fantastic way to make lots of money. You're a Bard, you have a high Cha (which covers 2/3 checks for gambling), lots of skills, and you have access to Expertise. If you can manage to get your bonus in at least 2 of those skills up to a +16, that downtime activity just prints money as long as your DM will let you get away with it.
You're a Bard, you have access to some pretty helpful spells. When hanging out somewhere and figuring you won't need spell slots, offer your services. Note that the official rule for "how much is spellcasting worth" is "No established pay rates exist"