Who else is obsessed with keeping your categories at the golden level???
I literally spend so much time just keeping my categories up. I almost never go on to the next one because I have so many already unlocked. Its crazy I know. But there is some crazy satisfaction having your thingies being golden!!!
This is my favourite method to get and keep 4 trees gold every day
I'd really advise you to keep progressing, even if some older skills loose their golden shine. I'm also not saying you should rush head over heels to the end of the tree, but try to strike a healthy balance between reviewing old skills and doing new lessons. With learning any language, it's important you keep challenging yourself.
My strategie with French was to keep all skills at least at 4/5 strength while I was progressing through the tree. You could also choose for keeping everything at 3/5. It doesn't really matter, in my opinion; the most important thing is that you keep going back to old material once in a while but new material is more important.