Signature by both taxpayer and spouse: I suppose you could have just the taxpayer sign in the absence of clear guidance but there's joint and several liability on a MFJ return anyway.
Safe Harbor: With 50-75 properties, it sounds like there's a possibility these could rise to the level a §162 ToB. Have you review the facts and circumstances to determine whether safe harbor is necessary?
Aggregation: If the taxpayer opts to apply §3.02 of Notice 2019-07, IMHO, election to aggregate under §1.199A-4 would not be necessary or appropriate as §3.02 would treat all similar properties as one single rental enterprise and one trade or business. See a recent discussion on this topic here: https://accountants-community.intuit.com/questions/1836204-cannot-see-if-this-was-answered-yet-if-ag...
Still an AllStar