The new way of learning isn't working for me
(I know the 'new' tree type isn't really new, but I haven'y been on for a while so I'm calling it new :) ) I think that the new tree type is good in the way that it gets you to practice old vocab, but for me it just doesn't work. I'm a bit of a perfectionist so before I move onto a new subject, I like all of the previous subjects to be gold. This means that because now there's so many subjects that aren't gold, I'm not really moving onto anything new. Most of the new lessons in previous subjects I already knew (because I learn languages with lots of other resources) so when I do a lesson in a previous subject I'm not really learning anything else either! Is it not working for anyone else? ;)
How long have you been away? The skills don't decay very fast, so I think everything will be fine after you restore your current skills to gold.
In your case, I'd recommend the following:
Practice all your skills that have decayed until they are gold again. There is really no reason for panic! You only have five such skills and you don't have to redo every lesson in them. Try "Practice skill" button. Sometimes it does not work to make the whole skill gold, in this case repeat the lesson that has decayed the most.
Throw in a session or two of "Practice all skills" every time you study. You can start with practice and then go on to learn something new.
My own experience tells me that if you do a lot of practice your skills may stay gold without decay for a very long time.
If you use the "practice all" feature, it will often strengthen the weakest parts of your tree without forcing you to focus on one section that you've mastered. If there are new lessons added to the program since your last visit, you can just test out of the entire section at once. But as you said, you've taken an absence from the program. I think the tree is made to "decay" with the assumption that you use Duolingo as one of your primary tools of learning. In more regular use, the earliest sections never decrease, in my experience.
I guess what I'm saying is that if you're concerned that you're going to have to constantly repeat easy lessons to upkeep all the branches of your tree, that won't be the case if you use Duolingo daily. I understand the perfectionism all too well. :)