For openings I highly recommend you check out
chessable.com and browse what they have available to see if any of the openings match. Some of their free content may overlap as well. Training on that platform is very easy with spaced repetition and you can even set it to not quiz you on full lines (I believe, anyway) in case you end up with a book that is too detailed to learn fully in the next couple of months.
For chesstempo my recommendation is to do standard problems, not blitz. At the very least until you are at problems where you need more than 5 minutes to solve them. This will help train your calculation a bit too and in a real game you are not really under blitz time conditions - getting as many problems right (or all problems right) should be the goal, not how quickly you are solving them.
Having said that, at some future point solving for speed may make a lot of sense.
I'm not even sure going through HTRYC is even useful - I went through some of it many years ago but my memory is that it focuses a lot on positional stuff and long-term planning - things I'm not sure you can "brush up" on. I think something better than that would be to go through an actual game (in one of the openings you plan to play) between high-level players and guess the moves for the side of the opening you play. The idea would be to write down all main variations/ideas for each move that you have - perhaps using a chess clock or timing yourself in some way somewhat similar to game conditions.
If you have annotations for the game, that's great as you can compare your thoughts to them. Even if you don't you can try to understand why the GM played his/her move and how it may differ from your moves - and also seeing other ideas from the computer. The idea is to simulate the skill of playing in an actual game and also to try to "see more" - ideas, tactics, positional elements, etc.
However, this sort of training is pretty time consuming and fairly demanding. If HTRYC seems more "fun" or "doable" it almost surely will be more useful - it's far more important to do something that you can fully immerse yourself into and focus on hard.
What Dutch do you play? I play Classical Dutch but have pretty much no experience with the French or playing 1.e4 fwiw.