I bought a car that was only driven around the block twice a year for the last 10 years.
Algae in the tank.

I can't give you advice on the mechanical technical stuff like popping the injectors or whatnot., but I would just keep pulling that secondary filter and flushing it.
Also, try 1/2 quart of marvel mystery oil in the engine oil. It may free up some stuck lifters or clear out some plugged channels.