Should I keep giving cards when I'm King 13?

Solution 1:

New cards come out time-to-time and require gold to upgrade, so donating cards for gold could be worth it for upgrading new cards quicker as they are released (meaning, you'd just have to wait for getting enough cards instead of cards AND gold, since higher upgrades could get costly).

If you are already sitting on a huge pile of gold (from either stockpiling or maybe you spend enough real money on the game to accumulate) then the only other thing would be to show support for clan mates (and maybe they could be more willing to trade those future new cards to you if you request them, if they care enough to remember).

But if you have plenty of gold for future card releases, don't care about star points for cosmetic upgrades, and don't mind potentially being seen as a free loader from your clan mates (no judgement here)... then there is no real advantage to donating cards once you reach King level 13.