My buddy had the exact opposite happen with a company here in town. He booked one of those cheap deals and the guy walked in, looked at his furnace, and said the ducts were too far gone and tried to sell him some $800 air scrubber thing on the spot. Total bait and switch. So the fact that your guy actually showed you the camera footage and the junk in the bag says a lot. That shop vac bag detail is the real tell too, because a scammer isn't going to pull out half a bag of crud and then just leave without pushing a bigger sale. Sounds like you found one of the honest ones, they do exist. Old houses especially are going to have decades of dust and pet hair and who knows what sitting in those lines.

you look like you needed to hear this" is such a simple line but it hits hard because he noticed you first. A lady at a laundromat once told me "you can't fix everything today, so just fix one thing" and I still think about it like 6 years later. We never even traded names, she just said it while folding socks and left lol. Sometimes the randos know more than the people we pay to listen.