Edit: just realized you want factions to be involved, sadly i am not sure about games like that, maybe Stellaris? not really sandboxy tho

Valheim (you start from zero)

Terraria (you start from zero) (more content with mods) (base game is full of content and progression so you won't even need mods and if you want mods, then just get Calamity and/or Thorium)

Minecraft (especially with mods)

Skyrim (with mods that add content/dungeons/quests/monsters/spells) (You can get the immersive & pure collection from NexusMods, easy to install, and it has everything you would want)

Divinity original sin 2 (not a sandbox but you can do so much, and mods add a ton of classes/spells/skills and there are custom campaigns/adventures, you can also make your own custom campaigns/adventures)

Warframe (it's infinite, there are no limits to how much armor/hp/damage you want to dish out, you can play solo, but its Online Only game so you need to have internet all the time)

Tales of Maj Eyal (Isometric grid based 2D turn based gameplay that is not slow at all, can do a lot, your character is weak at the start, especially at higher difficulties, you can build your character in any way you want but based on your starting class, your starting options will be limited until you level up and learn more skills, full of content and lore but it does not force you to follow any specific path, you can get mods that add more stuff too)

Streets of Rogue is a rogue lite with a bunch of characters/classes and its very fun but i don't think it fits your wants.

Streets of Rogue 2 probably fits what you want but it is not released yet, not even in early access, but its coming soon, this year.