Ok, this is proof-of-concept level, so not sure how much of it is worth pursuing yet, but here you go: https://imgur.com/a/3fN3hqL

My motivation:

Bottle for longer, hotter rides around town when I mostly know where I'm going
Bag for very long rides when I need more calories - I don't normally want it there interfering with my knee range-of-motion.
Phone for any other time, partly with the idea of being able to snap it off quickly for a photo

Bottle: the off-the-shelf bottle mount works pretty well. My top tube slants down to the seat, so the bottle is inverted, and I thought I wouldn’t trust a normal bottle cage there. But to be truly micro-optimized, it needs a “self-sealing” valve like on all my other bottles, and this Peaty brand bottle doesn’t have it - but Fidlock bottles do.

Bag: attaching the mount without puncturing the bag makes it clunky, but it works - way better than emptying the bag and using a wrench to remove the screws, then reattaching something else. The mount adds like 2cm of height, but it still fits my bike pretty well. If I commit to this, I can make the mount a bit more compact. I bought the Apidura bag when I got this bike, because it was one of the only bolt-on products I found. I like the design of the bag, but since I’m attaching a mount to it, a strap-on bag would work just as well. I haven’t yet looked for one with an integrated phone display pocket that I like.

Phone: functionally, it works. I’m a bit concerned about knocking the phone with my knee and breaking something, because this mount is designed to be pretty sturdy if you don’t use the proper twist motion. Mostly, though, attaching this mount to the case is never going to work well unless the case is pretty thick (I’d use a router to carve out a space for the inner part of the mount). I haven’t been able to find a case that would work well. (alternatives in this space: peak design, quadlock, F3, SP-connect - but none of them will work for all of phone, bag, bottle).

This is pretty silly, but I’m also considering mounting my phone to the top of my thigh (SP-connect armband might work). I really don’t need this, I just feel like it should exist, so I want to see if it works.

Also, after doing this, I found these:

https://fidlock-bike.us/products/twist-essential-bag
https://fidlock-bike.us/products/twist-tool-box-bike-base - this one could perhaps be hacked into a phone case.

tldr: I don’t currently have anything that actually addresses your request (waterproof phone case). But if I find a product like that, which can sorta be hacked to work with the Fidlock hardware, I’d definitely do it.