Ooh my favourite subject!
Loads of options available with those figures.
The only watches that can be considered investment pieces at the moment are Rolex, AP, and Patek. And due to restricted sales they're out of most leagues new and inflated to silly prices pre owned.
Rolex if you can buy from an AD (authorised dealer) at RRP you'll be fine.
Even if you pay slightly over the odds for something like a Datejust 36 from a grey dealer which is an excellent choice for the female wrist - you'll be fine.
If you buy the following brands new at retail, you'll lose money fairly instantly
JLC
Omega
Panerai
IWC
But, if you buy those brands well and you buy pre-owned - your money is likely to be safe as they will have taken the initial "hit" from RRP.
To be honest, with your budget I would hands down be looking at "5 digit" Rolex sports pieces.
Rolex GMT Master II ref 16710
Rolex Submariner Date ref 16610
Rolex Submariner 14060 or 14060M
All of the above offer the chunk without being overbearing, and all of them are stable to invest in, but they're also all likely to be at the top end of budget or just over UNLESS you buy privately.
If you like proper chunk, then you cant go wrong with an older Panerai, either Radiomir or Luminor. The Radiomir has a 45mm case but is cushion shaped so doesnt look ridiculous and is popular with women.
You an also buy them new in 40mm which is the same size as the above Rolex I mentioned.
Image attached is of a 45mm Radiomir, you can see it's big but not obnoxious because of its shape.
If you like them (and they are marmite) see if you can hunt down a Panerai Radiomir 8 Days PAM00190 - it's an old model from when they were serious, but has an 8 day Jaeger Lecoultre movement in it so a top end movement in a chunky steel case that you can change straps on to suit any outfit.
It's also very good at keeping its value and can be had for 6-8k depending on condition.
If you give me a clue about what you like then i'm happy to make more suggestions! These ones are only based on my tastes.