Aww that's great to hear about the bigger tank
Just keep in mind another upgrade might be needed, depending on how quickly the fish grow
Generally my way of telling if a fish needs an upgrade, is judging by the size of the fish and the size of the tank.
So one stocking rule is that you want a tank to be 6x the length of the fish (I find this rule works well for goldfish). So for an adult goldfish this would be a minimum length of 6 foot, but with your fish being a lot smaller currently I think something 4 or 5 foot would do for now
You can then decide later whether you want to up the tank size (it'd have to be something at least 6 foot, to support the goldfish's growth as they never stop growing) or build/find a pond.
I'm in a similar situation as my goldfish is very little (probably a similar size to yours) so I have her in a 90L/3 foot tank, but as she is by herself, water quality and stocking isn't an issue, but once she's around 6 inches (which is roughly 1/2 a foot), I'll be upgrading her to a larger tank or a pond (which is where single tailed fish really belong).
Definitely seems like lack of cleaning could have been the cause of the pimple, I can only imagine the mess you came home to! My one goldfish makes a huge mess by herself, so I can only imagine what 4 of them would do!
Glad to hear your fish is getting better though