Coma is certainly another possibility, however (without throwing shade) I'd find my thesis more alluring. In any case, taking MC out of the happenings for some time makes sense for the classical hero's journey. Perfect time to lorebomb as well as have him train more AND become humble again, seems a pretty big deal this chapter was to emphasize how cocky he became, yet he would never stand a chance against Thanatos or the Founder despite his growth until now. Also allows the LIs to ground themselves and realign their relationship with MC towards a more harem focused path in his absence, depending on how dark you wanna go you could have them either mourn his death OR just have them clueless about his whereabouts. I mean with my idea at least the LIs could stumble upon MC halfway through his absence while being trapped in Eternum momentarily so we wouldn't miss too much interaction. Could also tie in more Calypso lore/arc into that. It would also solve the issue of MC having his body still in the real world while in Eternum if Cari desires to go down the story path of bleeding cases and MC being a traveller between both worlds further.