The subfloor needs to 10 perfectly flat, all the glue must be removed 100%. Any imperfections will telegraph thru anything vinyl. Secondly those type tiles are not going to be a good floor for a bathroom at all.

You have a lot of grinding ahead of you. The amount of glue left over is WAY too much for anything else that needs to be glued.

So you are going to be grinding, grinding, grinding until you arms ache. And then you will grind some more.

Once you have 97%-99% of the glue removed, you will then do your best to deal with the wonky-waviness that you have uncovered. You will grind down the high-spots and 'fill' the low spots with Self Leveling Compound (which is NOT self leveling BTW...it can be a bit tricky to get right). Once you have applied the SLC, you will then do a snick more grinding just to get it smooth.

Now you get to install your temporary peel-and-stick floor. The OLD peel-and-stick tiles were SUPER GRABBY. That was back in the day where NO ONE CARED about indoor air quality. Today we care a lot. Which means the glues have been 'toned down' to the point where they are more delicate than the floors they are holding down.

To get 'water proof' you need to work with sheet vinyl. That's your place to start.