The problem isn't that the galleon has "too much" to manage, it's that the sloop and the brig were made laughably easy to manage by a full crew because they also had to be designed to work with a minimum crew.
The galleon was the first thing designed for Sea of Thieves in the development cycle and has a very clear idea: A crew works a ship together, and they can all do one thing at the same time but if they try to do everything at the same time they need to split up.
The galleon is designed for 4 players, and you can have 4 people firing cannons or you can have 1 person steering while 3 people adjust sails. Then when it comes to repairing, you have to lose someone from the sails or the cannons, as you can't steer and repair.
The sloop, from everything that is publicly known, came much later in the development and was basically to deal with pushback that some people didn't like the idea of having to play with other people. So the sloop is designed primarily as a ship that 1 player can operate.
On a sloop 1 player can easily steer and adjust sails, the cannon isn't far from the wheel, and the ship takes on water so slowly that it's easy to steer/shoot and only nip in it bail/repair occasionally. It is fastest into the wind because it also means it's rarely in your interest to even move the sail, meaning you just need to steer/shoot/repair. Only 1 cannon per side, 1 sail to manage, and the capstan raises pretty fast with 1 person.
So you basically had two ships: 1 designed for 4 players to just have enough to work it, and one designed so a single player can work the entire ship easily on their own.
So what happens if you don't have 4 players but there's more than 1 of you? Well the answer Rare went with is the duo sloop, and "3 person galleon". The duo sloop is basically 'over resourced' in the way that unlike the 4 player galleon you CAN do everything at once. The three player galleon though just can't do everything.
So then the Brig was introduced for 2-3 players. Thing is once again it's been designed so 2 people can sail it. It has 2 cannons, there are 2 sails. It is very easy for the helm to nip down and repair or nip across to the cannons and shoot. When you have 3 players on the brig, the brig is over resourced, you can be firing all your cannons while someone repairs/steers just like a duo sloop.
Really the Galleon should be 4-5 players (So you have 1-2 = sloop, 2-3 = brig, and 4-5 = galleon). The reason Rare actually limited crews to 4 people is because they felt that when they tested larger crews they found that the crew split into two 'subcrews' and that's not what they wanted. They didn't want a situation where you had the helm and the cannons as a crew of 3 and then have a crew of 2 as the repair, for example.
Personally I think that so few players sail a galleon these days because of the over resourcing of the sloop and brig and the declining popularity of the game (naturally after 5 years) they should just increase the galleon to 5 players. The reality is a half decent galleon crew can sink practically every sloop and brig anyway, so there's no "balance" issue beyond galleon vs galleon if someone has 4 players vs 5, but thats the same as a 2 vs 3 brig or a 1 vs 2 sloop.
