Approaching this too casually?

Look at Weefine (Kraken if you're in North America) Smart Housing. To use it, you run an app on your phone, connect to the housing via bluetooth, and then use buttons on the housing to control the phone. It is made out of aluminum and rated to 80 meters, so you can use it for scuba diving. It also comes with a vacuum system, which significantly reduces chances of flooding - important when you use it with an expensive phone that has lots of valuable data on it. There is a version that includes a pressure sensor, in which case your housed phone doubles as a dive computer.

I'm not familiar with either of the cases that you referenced, but the Catalyst one doesn't look like it would provide much more than splash protection. AxisGo is made by Aquatech, a well-known manufacturer of surf housings, but it's only rated to 10 meters, so it won't stand up to pressures of a scuba dive. It also doesn't have a vacuum system to validate the seal.

Keep in mind that the high-quality underwater photos that you see are almost invariably taken with strobes. These strobes add significant weight, bulk and expense to your rig, and - this is the important part - phones, with their lack of a mechanical shutter, cannot sync with strobes, so if you take photos with a phone, you will be limited to natural light or an LED torch.

There is a relatively inexpensive housing for your X100F (link), but it is one of Meikon's older models which lacks a vacuum port or double main door seal. Also, the fixed 35mm focal length of the X100F is somewhat limited for underwater photography - it is neither wide enough for true wide-angle (and even wet wide lenses will be limited by it, as they typically target 28mm or 24mm focal lengths) nor long enough for macro with diopters. Housing your GX9 is also possible, but this route will quickly take you into the four figures and multiple suitcases territory.