Thargoids will never accept peace. This may sound a little extreme but I have a few reasons to believe so.
Thargoids are extremely aggressive species and don't want peace. For example, during the Thagoid-Guardian war, the guardians tried communicating with them but the Thargoids rejected them and only escalated the conflict. (Check the Elite wiki for more details)
Even if they were nice guys who want live and let live there's another problem – we tried wiping them out TWICE (Mycoid Bioweapon and The Proteus Wave)
Edit: Only one race will survive the war (and it will most likely be thargoids)
After you complete the campaign there’s sometimes a opportunity to do a harder version. You can somewhat replicate the difficulty by using a mod that ...