Those who prefer comfortable geo in a trail bike (rather than long, low, slack)

At 5’10” and with my proportions I can ride a modern large or a medium and there are small advantages to each.

My ESD has a 490mm reach and a 63 degree head angle, and is very comfortable while climbing. I can ride that bike all day. The reach is a hair long for me, but I only notice it when trying to pull into a manual.

My enduro bike is almost 10 years old now and has a 430mm reach (but a way slacker seat tube angle too). It has a 65 degree head angle which is about where the ESD rests when the fork is weighted.

I’ve really grown to prefer the more modern geometry due to the way they climb so much better. I even do a lot more out of the saddle climbing since there’s much more room. I would potentially buy a medium modern bike but there’s no way I’d size down to a small.

I'm not sure I understand how HTA is fitting in here. To my a comfortable pedaling position (which I do value as many here do) would seem to be sufficiently defined by where is your saddle relative to the bottom bracket (when in the highest position) and where is the handle bar relative to the saddle. Numbers like reach, HTA, stack height, blah blah blah all have some impact on these, but why not just describe the quantities that matter for comfort directly?

Take reach. So you have two bikes where the seat is in the exact same position and the handlebars are too, but one has a shorter stem and more reach. I get how one bike is going to handle differently, but I don't see why one would feel that different pedaling on a non rough, mild uphill path.

Or even STA (that I hear described all the time as super important down to the degree). It seems the rails on the seat offer some amount of change fore/aft and if the STA is too steep or shallow for you to get the seat where you want it that is going to be an issue. Does that happen to a lot of people? I ride a 2014 Salsa Horsethief and I've played around pushing the seat as far forward as it will go and it seems a bit off to me but I haven't done as much testing as I'd like yet to definitively say where I'd like my seat.

I am also a bit nervous about all these geometry changes these days. I'm going to get a new bike at some point, but I held off this year as it was just too hard to test ride stuff easily. I'm hoping next year will be better and maybe I can make it to some demo events.

As far as handlebar relative to seat goes, mine is also about level with the seat and the distance from seat to handlebars seems about right. I guess I would define this as what angle is my back at when I'm comfortably holding the bars - I don't want to be way stretched out and I don't want to be riding a beach cruiser either.

What are the core numbers to quote in terms of the way I'm thinking? It seems like it would be something like:
Body Height
Level Saddle and across the top of the saddle, how far is the perpendicular down to the BB and how far back is it (and you have to pick a will defined spot on the saddle - I guess wherever you think your pelvic bones are sitting
And then how far up or down is the handle bar from the saddle and how far away is it (perpendicular distance).

These aren't numbers you'd quote that are the same for a given bike obviously but it would be interesting to read other people's numbers translated like that.

I think that the newer frame geometry is influenced by the growing popularity of bike parks with big sweeping high banked turns etc. and lift serviced ski areas with Mt bike specific designed trails. I ride a 2015 27.5 full suspension bike and the geometry is somewhat tamer then the newer bikes. I rode a friends new bike recently with the latest frame geo, also 29 inch wheels, on the same trails I ride ( tight turns, heavily wooded PNW natural trails ) and found it to feel a little sluggish, could have been the bigger wheels. Defiantly felt a little smoother on the rooty downhills but somewhat slower on the up. I'm 5.9 and have always preferred a 17.5 inch frame my friend is the same height and his bike is a 18 incher. As far as reach and stem length I stick to the old rule that when you are in your preferred riding position your handlebar, at the stem, should cover your front axle when you look down.

Ass-hatchet.

First I have to give attribution to The Radivist for that term, but it is so good I have to steal it. But it is a real thing. Those who aren't riding hard tails might say STA doesn't matter. Oh, it matters. Especially on a hard tail. My ride for years (and I still have) is a Ti frame with modern-ish geo, but a pretty relaxed STA (74-ish) and longer chain stays (438mm). It rides like a soft tail. I recently completed a build on a Nimble 9, a beautiful steel frame with a 77 STA and is set up with 420mm stays. It is a reasonably compliant frame, but there is no mistaking which bike you're on. The N9 will give you quite a prostate punch if you hit a ledge or square edge when seated. It's all about geometry, that closer-to-vertical seat tube and a wheel right under your butt that gives you a good jolt. Now, imagine the problem on a burly overbuilt aluminum frame with similar geo....keep your proctologist on speed-dial. That kind of stuff beats you up on long rides and just hurts.

Full squish guys don't really need to worry about it, but the OP mentioned hard tails - don't let anyone tell you it doesn't make a difference! If I'm building a HT with a steep STA, I'm going to be a lot more choosy about the frame compliance, and tires size and pressure.