This wasn't explicitly stated, but assuming OwnerCo is a corporation? Owner receives a W-2 and not self-employment income?
You cannot "treat" deferrals as catch-up by election; you have to exceed a limit.
$20,000 exceeds the 401(a)(30) limit by $500 so we have $19,500 deferrals plus $500 catch-up.
$19,500 deferrals + $5,500 PS = $25,000 which exceeds the 415 limit (100% of compensation) by $3,000 so another $3,000 of deferrals can be reclassified as catch-up.
Ultimately we end up with $16,500 deferrals + $3,500 catch up + $5,500 PS = $25,500.