Had the same dilemma two years ago, a newer 9900 from eBay or have my printer guru replace the head.
I've had this second-hand 9900 since 2011 and for sure got my money's worth. There always seemed to be one nozzle that was an issue but the prints were still perfect until 2020. Despite several cleanings, including using the service menu, the head had it.
I didn't even go to Epson for advice and went directly to my expert after I did some on-line shopping for a second hand 9900.
I looked on eBay and found a couple of candidates though each would have taken a day or two to drive and pick them up. But after a call to me printer guru, the choice was clear: replace the head and have the printer serviced on site with the assurance that it would be good for a few more years. He told me that the replacement head was a new design and new part number and in his experience, a much more reliable print head.
Despite the cost, the best thing was not having to dead with removing the 9900 up a narrow flight of steps and then having to dispose of it and then bring a newer 9900 down those same set of steps....
It took about ten days to get the part and schedule the service call, but I can tell you it prints better than ever now, a good call for me!
Since the replacement, I've left it sitting for a couple of times for up to 4-5 weeks on its own. When I'm home and ready to print, I just run a cleaning cycle then it prints just fine. The newer head just doesn't seem to need cleaning as much as the old one did, even when I print canvas which usually leaves a lot of lint to need to clean out of the printer after a couple of days of running it.
I figure if I can get 2-3 more years for this repair, I'll be quite happy. Not having to run a bunch of new profiles, change to a different set of inks and best of all, not having to deal with the moving and disposal of my 9900 was a no brainer for me!