I use a belt case like this: Alesser For LG G5 G6 Belt Case Clip Cover With Kickstand Anti knock Case Shockproof For LG V20 K10 2018 Protective Phone Case|Fitted Cases| - AliExpress
In fact, I bought it for my previous phone, LG V20. But after my FP4 has arrived, I discovered that an FP4 in the standard case fits perfectly into the belt-clippable cover.
Given the FP4 popularity, chances are that there should appear some 3rd-party made belt-clippable cases or 3d printer designs, as mentioned above.
As for the biking with the phone for GPS. I have put this holder on my bike: New Bicycle Phone Holder Mtb Road Mountain Bike Stand Motorcycle Mobile Cellphone Support Bracket Mount Gps For Bike Accessories - Bicycle Racks - AliExpress
For a city bike, it would be cumbersome to put the holder on and off the bike each time you take it (ok, it’s a matter of a minute to do that, but still - makes taking the bike slower). But since the holder is universal, it suits all phones up to some size, you can suggest the company running the city bikes to have a phone holder on them - chances are, people would use that.