As per this, I was able to get the following to work:
[Unit]
Description=remote external drive share
Requires=multi-user.target network-online.service
After=multi-user.target network-online.service
Before=kodi.service
[Mount]
What=/storage/.kodi/addons/virtual.network-tools/bin/sshfs#le-pi3:/var/media/external
Where=/var/media/test
[Install]
WantedBy=multi-user.target
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Though mount -t /storage/.kodi/addons/virtual.network-tools/bin/sshfs le-pi3:/var/media/external /var/media/test from the CLI after boot and:
[Mount]
What=le-pi3:/var/media/external
Where=/var/media/test
Type=/storage/.kodi/addons/virtual.network-tools/bin/sshfs
both give me the "No such device" error.
I'm using an ssh config file and haven't done any testing with Options.
This is on a Raspberry Pi 4 (1gb) running 9.1.002.
After typing this, I see you already tried What=/storage/.kodi/addons/virtual.network-tools/bin/sshfs#[host]:[path] in #5, so maybe it has something to do with the fact I'm using an ssh config file and/or not using any Options. I'll run a few tests on my end to see if I can replicate the "read: Connection reset by peer" error.
Edit: As soon as I commented out my ssh config file and added the Options, I started getting the "read: Connection reset by peer" error. I was able to fix it by adding IdentifyFile back to my ssh config file. I should also note that the system doesn't recognize the hostname from the other Pi, so I also had to either use its IP directly or add Hostname back to my ssh config file, or I got the exact same error.