Out of 9 rentals, I’ve got a couple of houses where tenants are clearly taking advantage of the current moratorium. They have decided to pay less than the amount owed even though there is no reason for them to do so, ...

The need for either a Quiet Title action, or to simply wait 10 years (here it is 4), is because it was a tax sale/auction.

This is definitely a great actionable first step.

I would first send a letter to ALL (dont send out the letters selectively) the tenants with an excerpt from the lease contract regarding tenants liable for their guests actions. 

Language from my lease:ATTORNEY/COLLECTION FEES: In the event that legal action must be taken against Tenant to enforce any part of this Lease or applicable laws, Landlord shall be entitled to its recover costs and reasonable attorneys' fees incurred in connection therewith.

A good real estate attorney will be able to quickly advise on your best course of action in terms of formalizing your partnership.

Currently, I'm putting my income into an ETF and then when I find a deal worth taking action on, I would withdraw and invest. 

Replicate it.Take consistent action.

I know I will take action at some point just not sure when. 

Daily action, even for passive investors, will push you further, faster.