No module named pip is usually because I have Python earlier than 3.9ish and the vast majority of recipes expect Python 3.9 or later.

A virtual environment that removes access to pip certainly isn’t working as desired.

Here’s some things your outputs tell me:

• The system version of Python has a lot of stuff installed. So using a virtual environment is definitely worth the effort.
• Your activate script looks fine, on a casual read. (One of the problems we have ruled out is an empty activate file.)
• Python and pip are both installed and available on your PATH, at least before you activate the virtual environment.
• Your virtual environment is doing things - at least enough to break pip.

Having ruled out an empty activate file, I would check on what shell is running. Your activate script expects bash - a classic - but your SteamDeck terminal could default to something else.

I would also try tossing a 3 at the end of the Python and pip commands. In some situations it can help a missing command be found.

Try these:

which python
which pip
python -V
bash
which python
which pip
python -V
source .venv/bin/activate
which python
which python3
python -V
which pip
which pip3
pip3 freeze