Telling people who are not invited to a certain event that they can not come

Just don't bring up the ceremony - only send them an invitation to the reception. If they ask about it, what you said is fine.

HIs it a shift to covid numbers? Generally, this is only done if the ceremony is extremely small and private, like just immediate family, or no non church members allowed. Then you do exactly as you said. But if you are having some family and friends, not 10 as in private, but 20 or more, with a wedding party, then it is not acceptable. You invite everyone and treat and treat them the same, or do not invite them. No having 2 tiers of service.
No, we are having our closest family and many friends at the ceremony, and those of you we don't like as much only can come to the reception, or miss dinner but come after. There is no way of putting that that is not rude.

There is no polite way to invite people to reception only.

This is awkward all together...

If this is because of Covid, then just explain to them and they'll understand. If this is not because of Covid, then I'd rethink your plans, because it's very rude to invite some to the ceremony and not others. You can get around this by having only immediate family at the ceremony, or by finding a different venue that can accommodate everyone at the ceremony.

View Quoted Comment Unless it’s a private ceremony - parents, siblings, grandparents only. Otherwise it’s super rude.

My cousin did this and it was no big deal. Only the parents and siblings were at the ceremony and the rest of the giant family was at cocktail hour. We celebrated them and did pictures etc after they came to the reception. I believe on the invitations it said something about an intimate ceremony and that we were welcomed to the reception.