Honestly, if they have been properly cleaned and lubed and they're still scratchy there's not much you can do. They're a bit like Alps in that dirt and dust will have scratched up the surfaces of the board if it was used a lot while dirty.
In general it is recommended to look for good condition space invader boards, the same as for Alps, as the dirty ones will probably be pretty scratchy. I'm guessing you got lucky with your RT8255C+ one in that it had only gotten super dirty after it was no longer in use, so it didn't affect the switches much.