I've taken two contrasting approaches to this issue:
1. In my Varisia/Golarion game the Epic (17-20) PCs will* likely spend most of their time on high level Epic threats from high level Pathfinder AP adventures, Books 5 & 6 of various AP lines. Stuff written for PF 13-18 can definitely handle 5e 17-20 I think. With just Books 5 & 6 of Rise of the Runelords and Shattered Star I have around 40 sessions of well-integrated epic level play, and it should be easy enough to add material from other APs. I suspect stuff like Tomb of Horrors will still be useable at 20th level also. Failing that I can always homebrew around Kill-the-BBEG type plots...
*Group is currently 11th-14th level after 50 sessions.
2. In my Wilderlands sandbox I have an 18th level barbarian, Hakeem the Exalted Champion of Bondor the sword god, but Hakeem is still doing mostly paragon/Tier III type stuff, questing around (often incognito) while gearing up to face his Nemesis/opposite number Kainos the Warbringer, the Empyreal son of the war god Bane, in single combat. I solved the overpowered high level party issue simply by ruling that new PCs started at 11th (non-caster) or 8th (caster) so they would essentially be his sidekicks. I have other lower level PC groups in the campaign where other players get to be the Hero of the Story, but I found that it simply worked best to accept that Hakeem was the leading character for his saga, and not try to shoehorn in a bunch of other 18th level PCs.