Does a condo owner also have to fill out a TM30?

Thank you for your quick response, Manow. How is the procedure? Where to get/download the TM30?
Can it be handled online or do I have to go to Immigration?

The legislation applies to all foreigners who reside here as Non-immigrant or Tourist.
It doesn't matter if you are owner, renter or whatever. No distinction is made.

TM30 form only says who has to report the persons staying under the same roof and that this is the form with which to do that
But that does not exempt the foreign owner from reporting where he is staying.
Surely he may also have several properties in Thailand. Where does he stay then?

Probably because you've been staying at the same address all this time, people won't ask anymore, but that's a local decision. That does not mean that this legislation does not apply to owners. It also does not mention owners not and tenants do.

By the way, 18 years ago it was hardly looked at. It is only roughly in the last 10 years that this has been applied more strictly. Which is not to say that such legislation did not exist, of course. It has been there since 1979 and that is why you will also read many references to the police or police station instead of. immigration in those documents, because then there were only a few immigration offices and that in the places where many tourists came.

https://library.siam-legal.com/thai-law/thai-immigration-act-temporary-stay-in-the-kingdom-sections-34-39/

Section 37
An alien having received a temporary entry permit into the Kingdom must comply with the following:

.....
Shall stay at the place as indicated to the competent official. Where there is proper reason that he cannot stay at the place as indicated to the competent official, he shall notify the competent official of the change in residence, within 24 hours from the time of removing to said place.

Shall notify the police official of the local police station where such alien resides, within twenty–four hours from the time of arrival. In the case of change in residence in which new residence is not located the same area with the former police stations, such alien must notify the police official of the police station for that area within twenty-four hours from the time of arrival.

If the alien travels to any province and will stay there longer than twenty-four hours, such alien must notify the police official of the police station for that area within forty-eight hours from the time of arrival.

If the alien stays in the Kingdom longer than ninety days, such alien must notify the competent official at the Immigration Division, in writing, concerning his place of stay, as soon as possible upon expiration of ninety days. The alien is required to do so every ninety days. Where there is an Immigration Office, the alien may notify a competent Immigration Official of that office.
.....
In making notification under this Section, the alien may make notification in person or send a letter of notification to the competent official, in accordance with the regulations prescribed by the Director General.

You don't have to unsubscribe anyone.
Either they leave Thailand or they are registered at another address. If the latter does not happen, that is not a problem for you.