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I'd offer them a new alternate capstone every level beyond 20, instead of new class abilities (they still progress at the steady stuff). Choosing one seems to be a meaningful decision, even at that character level. This will lose steam after a few levels, though, after the more interesting capstones are taken.
Personally, I'd rather worry how to challenge them before the final fight. There are not so many CR 21+ creatures in the books, and they are usually supposed to be unique. Improving CR 20- creatures comes at the risk of the players' question "Can't we fight something epic?". And building level 21+ NPCs can be quite exhausting, unless you excel in efficient preparation (library of NPCs, building blocks, reskinning, skipping unnecessary details etc.).
Also be careful with the duration of your campaign. Every month comes at the risk of losing a player, so playing year after year might result in a campaign cancelled before the end. If you want to go beyond 20, consider leveling quickly - unless your players are diehards with unwavering motivation, solid health and neverchanging private situation.