Watches

1. Breitling Aerospace Evo- I know you said no electronics or quartz, but honestly, this is one of the few exceptions IMHO, especially with his career.
2. Breitling Navitimer- Slide rule, nuff said.
3. Rolex Milgauss- A little hard, since they stopped production this year.
4. Omega Speedmaster- Cool, but personally, I don't like manual wind watches for everyday wear.
5. Grand Seiko SBGA 413/415 Shunbun. It's titanium, lightweight, and an amazing daily driver. I have a lot of watches, and this gets the most wrist time by far.
6. Tudor Pelagos- Titanium as well, and has a little more wrist presence, color, and lume.

These are all in the 5-10k range, but the aerospace can be found way cheaper used. Here is my list for the 1-2k range.

1. CW 12 in titanium.
2. Farer Portobello- Yes, a manual wind chrono, but I like the brand. sue me.
3. Upcoming Studio Underd0g series 2 watches (release date Nov 1). It's cheap, if you can get one, and it's cool IMHO. I want one, I'll decide on that day.
4. Longines Hydroconquest- I did like mine, I ended up giving it to my brother. Not my favorite watch, but value wise, it's nice.
5. Seiko Samurai SRPE33- Save the Mantas- This samurai, is a little big, but it has a ceramic bezel, sapphire glass, and a 3:00 crown and date with cyclops, vs a 4:30 crown and date. Amazing lume, a textured dial- Only downside to me is a 3 hz movement, but for that price.... It's a cool watch.

One last one, IF the regular Pelagos is too pricey, then possibly the FXD. better make sure he's a nato strap fan first before exploring this option.

Get something with tritium. Look at Ball.

The obvious and beste answer would be a Omega Speedmaster Professional with a self winding movement - this matches all of your requirements except the price. A watch that is about the same spec but cheaper would be a Hamilton Jazzmaster Performer Auto Chrono

Longines Big Eye Chronograph or something from the Longines Heritage line, maybe the tribute to the Czech airforce watch.

a vostok amphibia would be perfect in my opinion.

they're incredibly cheap

you can get models that look dressier, but they're still tough

they're automatic / mechanical

they're very durable

the movement is in house and the design is unique among dive watches, the soviets who invented it had to work with what they had, they couldn't use swiss designs because the soviets didn't want to pay royalties and so they had to engineer it well. the acrylic instead of glass is cheap, but it's also necessary to make it as waterproof as it is; it flattens as it experiences pressure, pushing it in and getting more waterproof. even the case back is different, instead of a standard screw back, it has a ring that screws in while the case back itself just sits under the ring, so that it doesn't ruin the gasket when returning from extreme pressure.

Poljot Strela 3133? Go the cosmonaut route.

Baltic bronze. Just picked one up at the New York watch fair. Can wear everyday but can still work as dress watch

Glycine Airman might also be a cool idea, worn in space and a Swiss Automatic

https://www.glycine-watch.ch/history

Check out behrens apolar. Similar theme but abit more expenssive

Without a price range frankly it's a little difficult, I have seen people calling a 5k watch a "beater" and people considering spending more than 1k a bonkers move.

In any case...

Divers are king of versatility, Water resistance and construction quality let you use them everywhere, but they are still classy enough to be worn with suit and tie, so I would go for one of those.

Orient is the budget option, the new Kamasu simply kicks ass. The design is not for everyone, though.

Oris is the best bang for your buck in the low end spectrum, a Aquis would be my choice but the Sixtyfive is a great skin diver.

Sinn also have great offering and it's the embodiement of engineering applied to watches, a 104 with a grey sunburst dial would be perfect for an everyday watch.

If you are willing to spend a little more, can't go wrong with a Tudor in my opinion. Given the versatility restriction I would go for a 58, difficult to do better with 4k bucks.

I would stay clear from Seiko, especially on the higher end. QC issues are common and some of the calibers are atrocious.

Happy hunting!

Casio calculator watch

Bulova Accutron Spaceview. Again, like the Lunar Pilot (which I love), it is a quartz, but with the tuning fork movement it's about as "engineery" and "spacey" as you're likely to find. That, and they're just cool watches.

I am a chemical engineer who has always been interested in space/astronomy, so honestly somewhat similar mindset as your brother since all of my owned/wanted watches are somewhat space themed.

I feel like the “Easy” answer is the Hamilton Murph like others have said. Interstellar is my favorite movie and I’m sure as an aero engineer your brother might share a fondness for the movie.

My last purchase was the new white Casio NASA collab. Super cheap but not sure if it was a limited run or not.

In addition I think a moon phase watch would be a nice little “space” feature, and one that I’ve had my eye on is the Engineer II moon phase from Ball. Pretty pricy but a step up from the Murph.