Actually, getting a deduction when buying from a legal entity developer is simpler than it sounds :) You can get it, and there are no special restrictions - the main thing is to properly prepare the documents. The deduction is calculated based on the full cost of the apartment, just like when buying on the secondary market.
For the declaration submission, you'll need: the apartment acceptance and transfer act, payment documents (a certificate from the developer about the amount, account statements, receipts), an extract from the Unified State Register of Real Estate about the registration of ownership rights, and a copy of your passport. If you took out a mortgage - add the contract and a certificate about the interest. You submit all this together with the tax declaration to the Federal Tax Service either through your personal account or in person.
You don't need to rush with the deadline - the law allows you to submit a declaration within several years after the purchase, so there's no real hurry :) The main thing is to collect all the originals and copies in advance, so you don't have to dig through the developer's archives later.