Just because parts are only five to six years does not mean they are functioning properly. It does increase the chance they are but the test is so simple. Plus some thing or things are not up to snuff as you have issues with the car.

Also some parts of the fuel supply system where not replaced remember. Because of your declining fuel milage the injectors may be checked. If bad enough they could impact starting as well.

Before that though since the car is a daily driver still I believe. After a run check your wheels for temperature. A dragging caliper or two could pull the fuel milage down. Another easy and cheap test. A dragging caliper heats the wheel and is not that uncommon. A simple comparison test from wheel to wheel. Always wet the tip of your finger in case you have a really hot wheel. Usually when that hot though you will smell it as well.

I do not think the additional chain stretch in only 20k could in itself be causing all the newer grief. Does the car still accelerate well? If a lot slower than it once was it could be a lack of adequate fuel now. Or at least less than you had once.

Getting the cheap stuff out of the way with simple tests is cost effective usually and you learn things that you will use in later years. No fun on the wallet taking a car in for paid service anymore. To add to the misery there are probably fewer and fewer mechanics still up to scratch on these older engines. You are somewhat fortunate though as there are some clues present. A 617 engine getting inadequate fuel will be actually harder on fuel. Perhaps not as bad as yours has become but a symptom not to be ignored as something is causing this. Personally I hope it is also impacting the ability to cold start easily as well. A not unreasonable assumption in my opinion.