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For the bike to securely hold a phone for navigating
I got one off Amazon, but at Swinley today, it creaked all the way round and bounced about a bit
I'd recommend quadlock to myself, but the phone is a Blackview and they don't make a case. I find the stick on quadlock mounts a bit useless
I ran a stick on quadlock mount for ages on an S7 with no issues. Only on a dirt bike though, where the fairing protects more than an MTB handlebar.
... i am being bombarded by adverts with Greg Minaar using a phone mount at the moment. Maybe ask him?
+1 for quad lock. I’ve used for years never failed!!
My phone has a textured back, so no stick on mount. It's got a case on, but it's only silicone, so too flexy and I don't seem to be able to find much in the way of hard, smooth cases
I'm leaning towards this one, but again, don't know how sturdy it will be

I’ve got that Topeak mount on my road bike so I could use my phone for navigation. I then got a garmin 530 so only use the phone mount on the turbo trainer to run Trainer Road. It seems pretty sturdy to me - but I’ve never tried it off road. Personally I wouldn’t want my phone stuck on the handlebar for off road where I crash from time to time.
Off road my Garmin now sits on the top tube using a project 76 stick on mount which seems really firm - if you could get some kind of phone case with Garmin tabs on the back that would be ideal to use with the project 76 mount.
I have a blackview 5900 and use one of these
It's been fine on the old school xc/gravel rides I do and means it's easily removed for photos. If I think it's getting a bit too rough I just put it in my jersey pocket.
I'd ideally get a proper quad lock type mount, but as you say with a textured and indented back they won't attach.
The Topeak smartphone drybags avoid the need for stick on mounts or dedicated cases and they don't need to be an exact fit to the phone like a dedicated case.
Not great for fingerprint / facial recognition but basic touchscreen swiping is fine and the topeak quickclick system is pretty solid.

I've used the Topeak and Zefal systems, due to differing phone sizes. The Topeak is better.