I’m going to offer a suggestion based on the “tempted to reset my progress” comment that you made - I’m assuming from the way that you’ve worded your statement that you have some reluctance, because you don’t want to reset progress on sentences/words that you already know perfectly well.
Therefore, I’ll put forward the suggestion to make use of the “Favorites” facility (which is “Favourites” in my Anglo-Australian world, but I’ll spell it “Favorites” just for you 
). You could do something similar with the Collections facilities, but Favorites has advantages, as long as you’re not currently using Favorites for … well, genuine Favorites.
I’ll try to give clear instructions for anyone else who might read this thread.
So firstly, you need to click on the Manage Collections button against the Fluency Fast Track (or any other collection that you might be studying) which is the button highlighted in yellow on this image -
Then select “100% Mastered” in the pull down menu highlighted in red on the following image, and click the green “Search” button -
- Now click the “* Favorite” button next to the "Apply to all on current page: " statement, as highlighted in yellow on the previous image.
- Then go to the bottom of the search page, and click on “Next” at the bottom right of the page.
- Now on the next page, repeat from Step 1. Continue until you have Favorited all of your “100% Mastered” items (i.e. when there is no “Next” button at the bottom right of the page.
Unfortunately, this will take a while as there are 20 items per page, and no options to increase the paging. But, that should change really, reeeaaally soon, as @mike said only 13 months ago 
that adding some paging options was on the to-do list -
Also, although we now have this excellent new Copy to Collection button available for each sentence if we wanted to create a new collection from these 100% Mastered sentences, there is no “Apply to all on current page” for this facility, so Favorites is easier to deal with from that perspective.
I see in your case @Dcarl1, that you have a little over 600 items that are 100% Mastered, so this will take a little over 30 pages to do the highlighting. It’s important to keep remembering to click the “* Favorite” button on every new page, as it’s easy to just eagerly swipe to the bottom of the page and click “Next” (not that anyone would really make such an error though, would they? 
).
Now that you have all of the 100% Mastered entries in your Favorites, you can go to the Favorites button and click “Play” -
The first thing to note about playing Favorites is that whether you get the answer correct or incorrect it does not in any way affect the Mastered status of the sentence. Therefore, if you get a sentence incorrect it won’t reset to 0% Mastered, and if you get a partially mastered sentence correct it won’t increase the % Mastered (e.g. getting a sentence correct that is %25 Mastered leaves the sentence still at 25% Mastered no matter how many times you get it correct in Favorites; you can only increase/decrease the % Mastered when it actually comes up for review in the main Clozemaster game for the relevant collection such as Fluency Fast Track).
Secondly, if you are someone who is interested in the point-scoring game, you should be aware that you don’t score highly for Favorites (only 2 points per sentence if I recall), but if (like me) you are not concerned about points, then this won’t be a concern for you.
Thirdly, each round of sentences in Favorites is totally independent of the previous round. What does this mean? Well, let’s say that you have 200 sentences in your Favorites, and you play 10 sentences per round. In the first round you get 10 sentences and you play them. In the second round you play 10 more sentences, but this may include 1 or more sentences that you just played in the previous round. This means that you may see some sentences frequently, and some sentences may not show up for quite a long time. You could increase the number of sentences per round to diminish this effect, but I don’t know if selecting “Infinite” would actually cycle through all sentences or not.
So, what is a good strategy to play these sentences now that you have gone through the pain of putting them all into Favorites?
As you play your mastered sentences, if you get one correct immediately you can just unfavorite it straight away by clicking the star, or on the web using the keyboard shortcut (Alt-F on Windows PC). This eliminates the sentences that you know perfectly well from any future rounds, and allows you to concentrate on those sentences that genuinely cause you difficulty.
For any sentences/words that you don’t know, you can decide to deal with how you feel is best. That could mean that you decide to do nothing and just keep playing them over and over in Favorites. You could decide that a particular sentence is actually quite difficult and reset it to zero, or it could be that you decide to create a separate collection called for example “Difficult sentences and words” and/or reset it to zero etc.
The good thing about this approach of using Favorites is that you don’t need to reset all of your mastered sentences to zero, just the ones that you genuinely have difficulty with. By that I mean that you could just look down the list of 100% Mastered sentences and decide which ones you think need resetting, and which ones you think that you know well, but using this technique with Favorites, you aren’t having to guess whether you would know the cloze word or not, because you actually do play the sentence.
Another advantage of using Favorites over creating a separate collection, is that if you make edits to the translations, add hints, notes etc. then these are edits that you are making to your version of the actual sentence in the collection (e.g. Fluency Fast Track). That means that when the sentence comes up for review in the main Clozemaster game against your collection (e.g. Fluency Fast Track), then those edits are all still there for you to see. If you were to use the Collections facility instead, and copy these sentences into a collection of your choice (e.g. “Difficult Sentences and Words” as I suggested previously), then any edits would only be to the copy that you had made, and would not be visible when you next played the sentence in the original collection (e.g. Fluency Fast Track).
Finally, outside of your current situation of the “temptation” you have to reset all of your 100% Mastered sentences, the Favorites facility could be something that you might want to incorporate in the future when you talk about listening vs. text entry exercises. At that point you could choose one mode (e.g. listening) and as you complete each sentence you could then make it a Favorite directly when playing the game, and then go and repeat the exercise for those sentences in Favorites for your other learning mode (e.g. text entry). Effectively you would get the chance to play both learning styles, but because Favorites don’t affect your actual % Mastered, you would get the full progression 0/25/50/75/100 for both learning styles.
You could even throw in the third option of playing the …
Anyway, that’s just one suggestion for how you might consider using the extremely powerful features of Clozemaster to deal with your current dilemma.
