Depends on your climate. In the Midwest we get lots of thaw freeze cycles and that wreaks havoc on pavers if you don’t have a good retaining system. The metal is fine for a bit but then after a few season your pavers start to shift. Concrete works the best if it’s deep enough.

You want to first direct any pooling water away from that area. Even if you do a base and pavers, if a lot of water collects its going to wash away the base and your going to get pavers that will start sinking. As for keeping the pavers retained, a concrete retainer would be best.

Your never going to get that crack to be water tight. If you can find a replacement tank get a new tank, but otherwise just swap out the toilet. It can be totally DIYed with minimal tools.

I'm not advocating for shark bit fittings, but at the end of the day a shutoff valve has to be in an accessible area. You can't close it up in a wall regardless if its a sharkbite or not. A shutoff valve has the potential to go bad, leak, and so on because its a bunch of seals and moving parts. A sweat joint is the way to go if your closing it up in a wall. You should check on your shutoffs once in a while anyway and "exercise" them especially your main shutoff. The last thing you want is a leak and not be able to shutoff the water because of a bad shutoff that wont budge.

We have one bathroom with an inline duct fan. We didn't realize there was no bathroom vent when we bought the house and there wasn't a great way to run a separate switch since everything is in conduit so the fan turns on with the bathroom recessed lights. The inline fan is so quiet I love it and will probably put in another one in our master bath once the current fan dies.