I have humvees with the liners that came with them. I found them ok for a while, but recently I’ve struggled with them and over a 40km trail centre ride ended up quite sore.
I’ve started wearing my DHB Lycra road shorts under the humvee baggy outer shorts and they’ve been so much better. They’re not wildly expensive and the longest ride I’ve done in them is about 50k off-road – dhb (and other brands) sell similar stuff with even better padding in that is meant to be better for all day riding.
I’ve not tried bibs but never had an issue with waist shorts staying up or digging in uncomfortably so not bothered either way. Plus bibs look ridiculous and I assume make it harder to go to the loo whilst out riding?
Edit – to add – if you aren’t using it already, chamois cream really helps on longer rides….
https://www.wiggle.co.uk/cycle/legwear/?ea=4294789716&o=2
In terms of sizing I went medium – I’m about a 33” waist measured – but I’m quite stocky. I found the large just had too much material in it for me.