This … is a troubling post. On one hand, adding a timer would just add a lot more stress and guessing to your reviews, which will inevitably make you get more wrong answers, and hate doing reviews, in my opinion, that is how I would feel anyway. By giving yourself time to think it over, you will either come to the correct conclusion on your own mental power which would likely solidify the answer more through the jubilation effect, OR, you get it wrong, you feel bad, but now you know the answer, it is up to you to get it correct from there one way or another. Sorry, for the run on sentences.
I also had a massive amount of reviews at one point due to an extended break, you are going to have to bite the bullet and get through them, 50 at a time, 100 at a time, however you choose to do so. As you say, you are very busy, it would be best, in this case, to slow down the lessons. You don’t want to quit, but running yourself into the ground will only end up forcing you to quit through frustration in the end.
My suggestion for you until you get less busy:
Don’t do any lessons for now, get all your reviews done to where you won’t be getting 100+ reviews a day, I will edit in a very useful script to see when you get reviews in a bit here: /t/Userscript-Dashboard-Progress-Plus/8309/220 If you go here, there are two amazing plugins to help you know what is going on with your reviews in the background. Specifically, the dashboard plus will show you your current level items and their progress to Guru status, then the ultimate timeline, this shows you exactly when your reviews will be unlocked and what items will be in those reviews. Unfortunately, it makes it easy to cheat, but it is up to you to hold back on that, it also notifies you of what review chunks hold your current level items which is pretty neat.
Once you have those guru’d/mastered/enlightened/burned, wait a few days, and do them again when they pop up. By this point, most of your reviews will be spread out in more manageable chunks through doing them in spurts when you can.
Now, after a week or two, you should have few if any reviews, now that you are over the big hurdle, do as many lessons as you feel you can keep up with, whether it be ten a day, or bigger chunks of 30-40 every three days, again, your preference.
Hopefully this helps a little bit.