Swords and crypts are pretty tough, and work well in goldfish tanks. Glad to see you taking a step in the right direction with the goldies. It's not likely they'll ever grow to full potential after being in the 5 for that long, so a 30 might do it long term for these guys. Don't plan on adding any more unless you can size up significantly though.
Please immediately banish any thoughts of minimal water changes. It's really not feasible in a 30 to set up a walstadt type tank with goldfish. They are immensely messy fish, and you will already be fighting an uphill battle when it comes to cleaning, on account of the substrate and plants hindering siphoning.
Please immediately banish any thoughts of minimal water changes. It's really not feasible in a 30 to set up a walstadt type tank with goldfish. They are immensely messy fish, and you will already be fighting an uphill battle when it comes to cleaning, on account of the substrate and plants hindering siphoning.