Those emails are E2E encrypted to another Proton user only. I’m assuming you used the web interface for Proton and not a Outlook type application where you bring in the mail and reply from that application. Or did you use public key/private key type encryption?
From what I read, Tor does not use the disk for cache, it is all in memory, so there is no browser history, search history, or cache to look at. OK, so the search company would know but from where and by whom. Remember your are on Tor. The forums would have copies like this one I am looking at right now.
I also felt like giving a barrister comment. That person may advise about your rights like can they force you to submit to a search especially if it is your personal computer. But a lawyer being a lawyer (via email) may not have advised you about wiping the computer as this may be construed as destroying evidence and there may be a legal issue related to that profession. This is Tor dedicated to protecting privacy, protecting identity, and opposing censorship. I’m not bound by that.
If it is a company computer then everything on it is theirs and, by the same logic, if it is your computer then they, de facto, gave you permission to put company info on that computer so is it yours.
Now I do not know what type of forensic search they will do. Will they make a non-destructive copy of the drive and then search that. I would want to be there when they do and want to see them destroy the copy when they finish. At this point it sounds reasonable to have a solicitor advise you. 
If it were law enforcement we would not be having this conversation. They would already have seized it.
In case you are interested in erasing, the site is: eraser dot heidi dot ie and you do not need the 35 pass option when erasing.
For anyone reading this and gets hired by a company where computer work is involved: let them supply one. Don’t use your own. Ditto for the phone.