Running

Plenty of the Garmin and Coros watches have great battery and screens for running maps. It will be as @doubleodavey says the maps feature you want. I have the garmin 945 and it has this feature but think the fenix models are better suited to it.

ooking at being more adventurous with Ultra style races and see that a lot of these events give you GPX map files to download and then follow on your watch to aid navigation.

Is there anything in particular to look out for in sales blurb of watches that can easily identify that they support GPX maps ?

Some I've looked at just say they are GPS watches which I think applies to ones like my forerunner and also to ones that use GPX mapping.

I've had a lot of Fenix over the years and bought an Epix 2 in February. A few random comments...

Epix 2 - I've used mine for 16 weeks of ski touring and mountaineering this year. Plus usual cycling, climbing and running. It has not skipped a beat. Battery life wise it sucks up multi day events. So far I haven't had to put it into the various low power modes to achieve this. Mapping is excellent. Have used it in whiteouts above the snow line and found it to be really useful. The altimeter was accurate enough to join in to volunteer some search and rescue activities in Austria this summer...does very well compared to professional altimeters. Its by far the best sports watch I've tried and I've tried loads over the years since 2005 or so. Its spendy and the main reason for being spendy is the higher resolution than the Fenix range. This also in theory compromises battery life although this hasnt been an issue for me at all.

Fenix 7: I chose the Epix because I was sure that I would need the extra resolution to make the watch usable for mapping and for day to day use when away from home for long sporting trips. Turns out I may have been wrong. I tried a Fenix 7 for maps a few weeks ago and was suprised at how good the screen looked. I maybe could have got away with buying a Fenix 7 instead of an Epix 2 and saved myself some money.

Tip for both: The sapphire option makes the screen more reflective and harder to read in daylight. I am also unconvinced that it makes the watch more durable. I;ve had the mineral glass versions of the Fenix 2, 3, 6, through to the Epix 2 and despite whacking them against rocks have never had a screen damage issue. I'd suggest that the sapphire upgrade isn't worth it. Garmin Pay works very well but virtually no UK banks support it. There's a list of banks that are supported. Check first.

Caveat: I've never tried any of the more recent Garmin running watches with mapping so if you don't really need multisport i'm sure there might be a better option for you than Fenix or Epix but they are very good options nonetheless. It does seem pretty clear that Garmin make the best multisport and mapping sports watches, its just a case of finding the one that fits your requirements the best.

seems like 'routable mapping' is the term for Garmin
https://www.garmin.com/en-GB/c/sports-fitness/running-smartwatches/?FILTER_FEATURE_SUBTRACKING_MAPPING=routable_map

I've got the Fenix 6 Pro and loading a GPX on is pretty straightforward. It'll show the route on the map and then beep or buzz when I'm approaching a turn so I don't miss it.

Forerunner 945 is basically the Fenix 6 Pro series in its triathlon variant
Forerunner 955 is Fenix 7
Epix is posh Fenix with AMOLED screen

(things like Forerunner 745 does breadcrumb trails, but not overlaid on a map)