The right amount of involvement may be less than the most you could possibly manage.

The most you could possibly manage is a strange place to set the level, and yet many people quietly set it there. Capacity gets read as obligation: if you could take the extra shift, host the thing, join the committee, then declining starts to look like choosing to do less than you should. Nothing held at that ceiling lasts. It also fills the space where somebody else would have stepped in.

Ask what amount you could still be doing in a year without dreading it, and offer that. Leave the remainder genuinely open rather than covering it quietly.