Proposal:

Instead of .bashrc, use .bash_profile. Bash non-interactive login can properly take those values automatically

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Instead of

Environment="PATH=/var/lib/pleroma/.asdf/shims:/var/lib/pleroma/.asdf/bin:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin"
ExecStart=/var/lib/pleroma/.asdf/shims/mix phx.server

Use just

ExecStart=/usr/bin/bash -lc 'mix phx.server'

Explanation:

Instead of defining path-to-asdf and set up a strict PATH, you can delegate bash to handle that. That means your modification to pleroma.service is just one single line.

I tried it the bash way before and it worked, but it set off alarms in my head that this could potentially lead to unexpected results. It's one of those "I don't know what I don't know" things.

At least for me, this is how the process is executed by Systemd

... Ssl  15:26  60:57 /var/lib/pleroma/.asdf/installs/erlang/23.3.4/erts-11.2.2/bin/beam.smp -- -root /var/lib/pleroma/.asdf/installs/erlang/23.3.4 -progname erl -- 
-home /var/lib/pleroma -- -pa /var/lib/pleroma/.asdf/installs/elixir/1.11/bin/../lib/eex/ebin /var/lib/pleroma/.asdf/installs/elixir/1.11/bin/../lib/elixir/ebin /var/lib/pleroma/.asdf/installs/elixir/1.11/bin/.
./lib/ex_unit/ebin /var/lib/pleroma/.asdf/installs/elixir/1.11/bin/../lib/iex/ebin /var/lib/pleroma/.asdf/installs/elixir/1.11/bin/../lib/logger/ebin /var/lib/pleroma/.asdf/installs/elixir/1.11/bin/../lib/mix/e
bin -noshell -s elixir start_cli -extra /var/lib/pleroma/.asdf/installs/elixir/1.11/bin/mix phx.server

Not saying A or B is better, just simplicity over complexity understanding this is still workaround instead of a final solution.

Going through the instructions as written, I ended up with lots of errors during the mix compile step, mentioning files that were compiled for a higher Erlang version.

I then removed the _build directory (in /opt/pleroma) and re-did the steps starting from mix local.hex --force, after which the compilation worked fine.