"Not sure the 4 day unpaid item thingo goes down well - buyers who end up paying get 'huffy' - then leave the 'huffy' feedback."

I have opened a few of those when the buyer doesn't make contact or pay. Mostly they do end up paying, even if it is on a date that suits them.
I have never had anyone get huffy. It doesn't affect the feedback they leave either, or they leave none.

Buyers won't have any legitimate complaint about having a UPI opened, as the seller is following the eBay process for UPI.