I Have tried small tires on 26" rims, lowering the bottom bracket. It does have a negative effect on geometry, significant enough to affect how well it behaves. You can compensate by increasing the trail on the fork but most would be rightfully squeamish at pulling that stunt.

Nobody said overinflating tires. A smaller more road tire on a larger rim leaves the geometry intact. The actual pressure is determined by the scope of the tire (the pressure range) and the weight of the rider. On the same tire my daughter can afford a lot less pressure for the same rolling resistance, my fat ass deforms the tire more under load and requires more pressure. If you want to be technical about it what you are trying to minimize with higher pressure is the road contact and your weight affects how much that contact happens at a given pressure.

I don't understand the constant push to put skinny tires on everything here. Refitting a 30 year old $40 MTB with a new 650 wheel set so you can run 32mm tires is the best course of action?

There is a whole category of high-performance, hard-surface tires in the same 26x2.0 that these old MTBs came with, so you can have high speed, good grip, AND the smooth ride of a big volume tire: They're BMX tires.

C&V might not be ready for that, but your kids are gonna love it.

Nice looking bike!
I'd buy really good tires and TPU tubes. Something like Continental GP 5000s 32mm wide and RideNow tubes. You will definitely notice the improvement in ride quality and reduced weight of your wallet. 😉

The condition looks great, I don't think mechanical upgrades would be that noticeable. Tires yes will made a big difference, a grippy tread with a supple sidewall.

I am, am, not sure about your cranks, those may be a recalled item. If so recall is still honored. I am sure someone will chime in with more knowledge on this.

Your bike looks great! Those tires are fine. You could spend more, or less, go fatter or skinnier, whatever you want. I think for your use I would keep what you have on there.
Bearings and chains are cheap and may make a difference if yours are worn. You should check your chain with a chain checker or by just measuring it. If your sprockets and cogs still look good you can keep them that way by making sure your chain is in good condition. A rusty chain should be changed. But, a new chain may skip or run rough on worn sprockets. If that happens you can run the old chain until you replace whatever cogs or sprockets that are worn.
Often you may not notice much difference in any one thing you do maintenance-wise, or replacing a worn part, but it all adds up. Making sure your hubs, bottom bracket, chain, & headset are all spinning smoothly, and your wheels are true, and don't forget the jockey wheels on your rear derailleur, will make your bike ride like new.
I think 70 miles on a mountain bike is impressive!

Is all about the intended use, there are just situations where they make the most sense.

We have old school mountain bikes (rigid frames, straight top tubes) we take to trails and river beds. The best tires are the fattest that will fit under the frame. For straight road riding a fast rolling tall tire that also fill the frame is best. On my neck of the woods there is just too much gravel on the roads so my 700c 28mm tires on my touring bike are beginning to feel too skinny, I'm hoping to bump them to 32mm. I'm building a new set of 650b wheels that will be 37mm, those should feel awesome around here.

Sometimes you are actually going fatter, sometimes skinnier, you should always hope to use the largest rim that will fit the frame.

I did have a beater mountain bike for a bit I slapped 650b tires on. When I sold it for $50 bucks it went with the old 26" tires it originally had.

How does one build tires?

(Asking for a friend.....)