Is it reasonable to ask my builder for a stiffer concrete mix under the slab?

The hosing-down thing is real, but it's more manageable than you'd think, and I say that as someone who just went thru this on our own build two years ago. What worked for us was getting the mix design written into the contract with the supplier named specifically. Not just '3000 psi concrete' but an actual mix number from the plant, with a low water content and a water reducer in it so the crew still gets a pourable mix without anyone adding water. That last part matters a lot. A soupy-looking mix and a low-water mix can pour almost the same if there's a plasticizer in it, so the finishers don't actually suffer the way your uncle's crews woud have back in the day. On verification: every truck carries a batch ticket, and you're allowed to ask the driver for it. It shows what went in at the plant and any water added on site. Stand there when the first truck shows up, ask for the ticket, and glance at the water added line. If the crew wants to add water beyond what's on the design, that should show up there too. A small amount on a hot day is normal and legal within the mix allowance. A hose running into the drum before it leaves the yard is the thing you're watching for. As for being that guy on site: it depends entirely on the builder. Ours grumbled a little but the pour went fine and the crew finished by early afternoon anyway. If the builder shrugs when you ask about the mix, that's a yellow flag worth one more conversation. Ask him to call the plant and get a mix with a water reducer specified. If he won't do that simple thing, I'd keep talking to other builders.

One more thing nobody tells you: the cure matters almost as much as the mix. Keep the walls wet or covered for the first week if it's warm out. Costs almost nothing, does a lot for long-term durability.

Another option if you don't want to play ticket cop all morning: hire a testing lab to send someone out for the pour. I did that on my shop slab and it was cheap peace of mind. The tech grabs a sample, casts a couple cylinders, and you get a break report later showing what the concrete actually did. Funny thing is, just having a stranger in a hard hat standing there changed how the whole morning went. Crew knew the deal, nobody reached for a hose. I'd ask the builder first though, since spring schedules get tight and labs need a heads up. If he balks at that too, that tells you something.

One thing I'd add since you're thinking on a 200 year timescale: the mix is only half the story, the other half is how much water you let sit against that concrete for the next two centuries. My grandfather's 1950s foundation is still in great shape mostly becuase the site drains and the gutters actually dump somewhere useful, while my neighbor's newer foundation is spalling like crazy because the downspouts discharge right at the wall and it gets soaked and frozen every winter. So when you're doing your site planning, spend as much time on grading and drainage as you do on the mix design, because even the best concrete will slowly suffer if it stays saturated through every freeze-thaw cycle.

Update: got the mix design written into the contract with the supplier named, like the first reply suggested, and the builder actually didn't push back much, which surprised me. Still chewing on the drainage point though. If the ground around the pour slopes the wrong way to begin with, how much of a headache is regrading after the fact? Trying to figure out if we handle that before the pour or live with it.