Call for a furnace cleaning , nothing else . Tell the new tech you are
keeping the unit, and just want it clean.

You said it isnt burning " brightly" do you mean it isnt a bright
yellow flame, or blue flame. Yellow is poor-cold-rich combustion, blue-
white is correct. The gas is in the chamber, right, and gets burned,
eficiently or not the only way you are in damger is if the exchanger or
unit is leaking Co, burnt gas, into the house. You did not say your
heat exchanger was cracked or Co readings were high.

4- 5 hrs to clean a burner sounds like bull. You got conned by a
salesman. So what if you have to get 3 more techs to inspect it. You
will find an honest one. If the Doc says you are dying you get a second
opinion, you need more bids anyway, your price sounds way high