So this worked on Mint before?
If so, what's changed on either end since then? Had there been an update to either Graphene or Mint since the last time it worked? Maybe Graphene improved a security feature or you need to reset the approval on the phone (maybe toggle the USB debugging setting).
I don't bother with MTP for file transfer any more - at best it's clunky and tedious, at worst it's unreliable. Better to use network.
Setup a share on your PC, and use Mixplorer or any other network-capable file explorer on Android.
Even better, use an app like Syncthing or Resilio, and you'll never think about file transfer again.
I have a Syncthing sync pair/folder for every folder on my phone to my home server, so I can manage files there and the changes sync back to my phone. This also provides a first step to backing up my phone. (Use Syncthing-Fork for finer control of each sync folder).
There's one folder I specifically setup for that random file I want to send to my phone. It syncs over any network, under any charge condition, so files show up nearly instantly (at most a minute later). I could tweak that sync folder to scan more frequently, but it's responsive enough.
Alternatively, using Resilio Sync you can selectively sync files from your PC, from the phone itself. The "Selective Sync" option is something Resilio has that Syncthing doesn't.
Both of these have an a phone and PC app. Install on PC, Select folders to sync and they'll index those folders. Install on phone, and select the local folder to connect to the sync jobs (folder) in the app.
I've found Resilio is hard on memory for the phone because it keeps the index in RAM, so I only run it when I want to grab files from my home pc. If you only sync a couple small folders, this isn't an issue. I use it for selective sync of my media folder (which is terabytes), so the index is large. This way I don't sync my entire media folder but can grab any music or movie file, from anywhere, and then I close Resilio.