Hero 7 Black if you can afford it. Rob did a video on best settings. Hero 8 is out but the extra feature dont really justify the extra cost IMHO and the new HyperSmooth is dare I say it just a bit too smooth for MTB and there's simply no feeling of speed or movement.
Chest or chin good. Chest gets more of an immersive feel, head you can obviously see where you are looking but most mounts put the camera a bit close to your neck in the event of an 'off'. You can get a cheap chest mount
here plus a load of other good bits (the GoPro one is good but hella expensive)
I'm no-where near as conscious of the camera on the chest, so it's like start and forget. When it's been on the helmet that's only when I wear a full face which isn't all the time.
Hypersmooth is amazing but it does chew your battery. You don't need a massive PC/Mac to edit it. Unless you really want it I wouldn't bother with 4k, it's just too heavy on any PC unless you use proxies. 1080p or 2.7k is still awesome.
The GoPro Quik app is perfectly acceptable but if you want something passable you can use Davinci Resolve which is free. Adobe Premiere or Final Cut for more pro feature but they are both expensive.
Be mindful of the fact you'll end up with a hard disk
full of video that you don't do much with..
This was all shot on a GoPro Hero 7 Black on my chin: