Well, given the additional information I'd first say that you've covered
the usual bases and then some.

Fuel injectors can be professionally ultrasonically cleaned and tested
for not much money so if it comes to that I suggest cleaning rather than
replacement. A Google search for "fuel injector cleaning" will show
what's involved.

Back to the lean miss. An apt description of what's slowly getting
progressively worse.

I suggest a through check be made for a small vacuum leak; the most
obvious site(s) being the intake manifold gasket(s) since the injectors'
"o" rings were previously replaced. But EVERY intake air connection
between the intake air metering device and the block is a suspect. I
believe that's so because the whole engine bogs down, not just one cylinder.

An exhaust gas analyzer can be inserted into the tailpipe to monitor the
idling engine's emissions while propane is carefully applied to each
suspect site. A vacuum leak will suck the gas into the engine, correct
its lean mixture and reduce its emissions.

If nothing is found this way and everyone is convinced that the spark
plug wires are OK then I'd suggest a compression test to determine if
there's a burnt (probably intake) valve. Let's hope it doesn't come to
this $$ level.