I’ve always found the idea of buying a watch with the specific intention of handing it down a bit weird, it seems a bit like trying to force faux-sentimentality onto an object for the intended recipient.
I sort of get the thought process behind buying a watch to celebrate a birth and then handing that down if the watch is worn and becomes associated with the wearer and the recipient actually liked it but otherwise not.
In this case it seems even more contrived, buy it, stick it in the safe, wait a few years and hand it over. Not only might they not like ostentatious displays of materialism, watches in general or whichever model you decide upon in particular, they may well end up feeling uncomfortable getting rid.
You can tell them they can sell it but if they think you’re emotionally invested in the whole ceremony they could easily just nod and smile and chuck it in the bedside wire drawer muttering about why didn’t you just give them money instead of something you liked and they’re stuck with.
I know what you mean about ISAs etc seeming soulless but you can at least allocate one ISA to something they “want” whether it’s a watch or travel or whatever and then it’s up to them without strings attached, perceived or actual.
If I had to buy a physical “thing” to hand over for that sort of money I’d probably just buy gold which most people would be fascinated with and can easily be cashed in without risking causing offence etc in the same way.