Hand of Zul and Shadow of the Empires both accept trolls from (almost) all non-Horde tribes, and are themselves not associated with the Horde.
Painted Shields take all tribes as well, though they’re mainly about Zandalari. They’re also more closely tied to the Horde, though I’m not sure to what extent.
Sandfury Tribe are not associated with the Horde, but obviously only take Sandfury.
As a guildless non-Horde troll myself, I can’t recommend doing it without a guild. While I haven’t been to Orgrimmar IC myself (yet??), I don’t get the impression sand and ice trolls are received too well there. While that can be a fun challenge, it can just as easily be an unnecessary obstacle. Easier to play a Darkspear or Revantusk, as I often wish this character had been, or to join a guild so you’re easily able to RP regularly.
It’s definitely possible though! Several of the guilds I mentioned RP in Zuldazar fairly regularly, and you’re usually fine in neutral hubs as well. Goblin ports especially. There’s also the Terrace of Tribes discord, advertised on these forums, which people use to look for troll RP sometimes.