I think from a prose-craft perspective, Howard is one of the best among his era of pulp writers. But Conan is clearly the person Howard wishes he was and is not a very useful character for trying to figure out what fantasy TTRPG heroes should actual play like -- not least because of the ensemble nature of the hobby. Conan rarely has peers (although he does occasionally; Belit comes to mind), and most of his adversity exists solely to showcase just how awesome he is in the end.
You're right, Conan is usually a lone wolf hero, which is not a good basis for a TTRPG where it's unusual for most of a group of players to be willing to be the sidekicks of a main character.
REH did have some actual peers for Conan. Belit is a good example, though she's quite the mess in terms of being taken by greed and suspicion. Valeria from
Red Nails is a very good example. She's got a mind and reputation of her own and doesn't just swoon for Conan... he has to
work for her. (Sadly she gets badly used by REH in the two bondage scenes... like I said elsewhere, product of its time....) In
Hour of the Dragon, the slave girl Zenobia doesn't get much time but she's certainly got serious pluck, so much so that Conan remembers her from the very beginning all the way to the end and she becomes his queen.
Conan was arguably the biggest single influence on OD&D, and even D&D has never been able to do Conan right.
It was stated as such, but I'm not sure it totally was because OD&D never emulated Conan in a meaningful way. You're totally right, they didn't get it right, ever. I think D&D was influenced by a lot of sources but never transcended its wargaming roots, not in the way that a lot of more modern games have.
I actually think Modiphius could have if they'd made the system less crunchy overall to keep the pace high and tried to make Conan a bit more like the first Schwarzenneger movie, where Conan, Subotai, Valeria, and Mako are a damned good adventuring party in a pulp context. None of them feel like pointless sidekicks and they all really need each other.
I think the dirty little secret of a lot of game companies, certainly of Modiphius at the time, is that many games aren't really written to be
played but to be
read and savored by people who have disposable income.
STA certainly had this issue, although it's escaped into the wild of actually being played. The 1E main rulebook was
rough to use in play due to its poor organization and white on black print, but it sure looked cool!