Problem

I need a way to transfer files to and from my Android phone in bulk. The main purpose is to update my music directory.

Initial attempts

Updating music via a find command with an -exec adb push {} \; argument overwrites existing files and so is suboptimal.
I got fixated on the idea of mounting Android as a filesystem. This was I tried various mtp tools, such as mtpfs, simple-mtpfs and go-mtpfs. I received broken pipe error messages even with small payloads. Then I tried mounting with gvfs. This was so slow it was unusable.
I realised that mounting was just adding additional complexity, and rsyncing over a network would give precisely the desired behaviour.

Rsync

In Termux:

pkg add rsync openssh

adb push ~/.ssh/id_rsa.pub sdcard/

Then, in Termux, move to .ssh/

mv /sdcard/id_rsa.pub .ssh/

Append to known hosts.

cat .ssh/id_rsa.pub >> .ssh/known_hosts

To ssh in via usb:

adb forward tcp:8022 tcp:8022

ssh localhost -p 8022

Rsync to phone:

rsync -av -e 'ssh -p 8022' ~/archives/music/* localhost:storage/shared/Music/