It’s all good, especially now that they have proton.me when it was protonmail.com I had some issues saying it over the phone as some people didn’t understand and it is long to spell.
The computer probably has local security tools (such as an edr) that spy on you any way.
You need to assume it is completely compromised.
But… assuming this isn’t in violation of your company computer usage policy (which it very much might be and can put you in trouble) you can install any VPN (avoid spyware shit) and a different browser (ideally something a bit obscure, like librewolf) and this will bypass the MiTM as the the device that does the MiTM would be either:
A) a network device that hijacks the HTTPS requests (VPN bypass this)
B) the browser used by the company
C) some other kind of software that atteches itself to all browsers via admin installed extensions (obscure browser might not be recognised by such software, be sure to check the installed extensions after letting the browser run for an hour)
And once you are done you can check the certificate chain in the browser to confirm.
Put an nfc tag there, you can insert it behind the leather. Write the same key on it and that way you could tip your hat onto someonea phone for a quick transfer for later communications.
With the way you stitched it, you could easily push one of the flat flexible ones there without having to mess with the stitches