How do I complete my 90-day reporting in Thailand without a rental agreement or TM30?

Just go to an agency

rent a nice hotel for couple of days and get TM30 report from them to take it to immig.....make sure to stay in the hotel till you get your 90day reporting done....

Do with an agency. This one you can do on-line 500 Baht. Do not need TM30. It takes about one week the process:

You have to do a TM30 within 24 hours of arriving at your resident. It is a legal requirement to report your location to the local immigration office or police station, every time you come back to Thailand. In some provinces you have to do this when you spend the night somewhere else in Thailand and come back, too.

If you haven't done it and try to do your 90 day report, you might get fined, then have to do a TM30 and then you can do a 90 day report.

Regardless of what you're planning, get a TM30 asap.

Your girlfriend can make a sublease for you, then use an agency they can bypass the missing tm30

TM30 is a landlord responsibility, but you are reliant on it. It doesn’t matter how formal or informal the arrangement is, you don’t need to be on the lease, but the landlord does need to lodge one within 24 hours of a foreigner’s arrival and they have to give you a copy on request. In practice, if they have never had a foreigner stay before, you may need to prompt it by asking for the copy. In practice there is no consequence for them if they lodge late, and they often have to lodge new ones to correct mistakes (since there is no “edit” function) so you shouldn’t have any problems even if they haven’t done it yet and have to do it for you now - Immigration sees ones with very recent lodgement dates due to error corrections all the time. But you do need to get it from them. If there are errors on the form, like a wrong arrival date or something, don’t try to correct it yourself (because it’s not your form), take it to immigration as is. If the error is serious enough that it needs fixing, Immigration will tell them what to fix.

But you should have done this within 24 hours of entering Thai land, and living in the condo. Immigration might fine the landlord 1,600 baht for a first offence, which of course you will have to pay

Your Girlfriend has to get her landlord to fill in the forms to say that you are living in that condo. You might be able to get her landlord to register online for the TM30 it’s much quicker and less trouble for the landlord once it is set up.