My magento FPC cache keeps getting reset. Why?

Hello, I am on magento 2.4.2-p1.

I have a good amount of traffic, but I still see pages getting non cache hits. Let me explain. I built the cache for the first 4 products of a category. And it was fine. I then had a hunch that changes in backend are causing the FPC cache to reset.

So I updated the title of a random product. I then went back to the front end and all cache was lost(not getting FPC hits). I built up the cache by clicking a few products and ran another test: I took a random product in the backend and added it to another category. Result: cache was reset.

So this makes me wonder, what else resets the cache? Please note, I am not manually resetting the cache in backend. Is changing the inventory of an sku going to reset the fpc cache? Is my store just broken? If this is normal operation, it makes me wonder what else is resetting the cache?

We constantly work in the backend adding products and updating descriptions from 10-5PM. But I also notice resets after 5 o clock.

Please let me know if this is normal for m2? Maybe I need varnish or just need to know what not to do to keep the fpc cache in the system. Please let me know if there are any solutions. My m2 without cache hits gets a ttfb of 1000ms, so this is very important to me(I am small business, not a developer, but I do have a developer team that doesnt know what is going on when I asked these questions).

I never thought I needed a cache crawler, as I had a good amount of traffic (2000 visitors daily). But if this is the solution everyone uses, I can get it, as long as it is smart enough to recrawl after a cache is cleared.