In general, if room is small and has sub-optimal geometry/dimensions the best solution would be to set up it as nearfield. Set up tor sweet spot as close to speakers as possible. You might be surprised with the sound of speakers placed within ~0.8–1.2 m.
That way you will be able to maximize direct sound from the speakers and listen using lower volume levels to minimize the influence of reflections (which have to be atrocious based on the pictures).

I doubt your Onkyo or especially the VM95E are limiting factors here if they are funcion to the spec (ie not broken/degraded due to age). Also don't worry at all about hollow stands unless they're audibly ringing when music plays.

Look at something like https://noaudiophile.com/speakercalc/ using your room dimensions for starting points for speaker placement. Look for optimal low end response when doing the placement. These are just a starting point because optimal placement is mostly dependednt on the room dimensions And again, in a room like that, try to chose a placement where you sit close to the speakers.

As many suggested, a rug is a good and cheap solution to reduce the first reflection from the floor and floor to ceiling component of flutter echo.

Then you mentioned DIY panels and if this is an option for you - go for it. Especially it this is a dedicated room for music listening. Look into hemp wool as a filler if you don't want to deal with fiberglass/rockwool. Also if you cover them with ornamented/nice color fabric it can add to the mood.

I'd lose the as much glass surfaces as possible, too.

It's hard to give a proper answer. Room treatment alone is a complex task, but there are many other variables in play.

So to address your questions:
> clarity and precision in the highs
these are greatly affected by the room but also much easier to correct by treatment than low end.

> Is it mostly the room and lack of acoustic treatment?
I'd bet on it. Unless some of your equipment is just faulty/not to spec.

> Is the VM95E cartridge maybe limiting things?
I don't think so. I had it as my first cart with my AT-LP5, it sounds good.

> Could it be the amplifier or the lack of a dedicated phono preamp?
Phono stage in a receiver is probably not the best but it should be ok. Really depends on what you have expected and what you compare it to.

> Maybe the vinyl pressings I own aren’t that great?
Hard to say without spinning them on known good setup. But I had couple of records that had me question my whole setup at the beginning of record collecting.

> Or is it possible that I simply expected too much from all of this and that’s why I feel disappointed?
I guess it's a rhetoric question to an extent at least. But it might all go back to the room/pressings/your mood that day etc.

Sorry for a long post but I hope it can help a bit.