Get a flat rate MLS listing service and you should be ok depending on how COVID is affecting your market now and going forward.If you're not comfortable with doing the above, hand it over to a realtor (one who knows their stuff) and let them do their thing.

For this one, you could ask about a flat fee for a broker to go over your documents and facilitate the rest of your transaction since you've done most of the work yourself already.

Can someone explain to me the different advantages and disadvantages of working at a brokerage with a percentage based commission vs flat fee?

We quote all flat fees with my firm.

If they are giving you flat cost for everything, percentage draws are pretty much guaranteed to get you behind and not leave enough on table for contractor to finish job.But to your question, it is amazing what interviewing other people will do for contractors that think they might lose the job. 

Has anyone utilized Rex Real Estate (2% flat commission, rexhomes.com)? 

I didn't know what to use.I'm currently using a property management company charging 100 flat fee on all SFR.I used zillow for the taxes.

Two six flats. 8/12 tenants now not paying rent.

So, if the budget goes up, the flat tax rate goes up.

Flat is much better then dealing with slopes