All of these are classic roguelikes, a genre of games which frequently aren’t much to look at. The tradeoff for the looks is that they offer vast depth and complexity… and (usually) permadeath and a learning curve that’s more of a cliff. I recommend watching some yt videos about any roguelike you want to learn more about, just so a fan can explain the appeal and show off all the basics.
That said:
Caves of Qud - actually one of the prettier classic roguelikes, if you can belive it. You’re a traveller in a strange and unique world of vast salt deserts, jungles, and the titular caves. There is a ton of flavorful, semi-randomly generated history (especially the ever-important tales of the sultans) and cultures, so every run feels different. There is technically a main plot, but you can just ignore it and go exploring - it’s a sandbox experience. The best parts, to me, are the aforementioned flavour, the tactical combat (that can get incredibly chaotic, with screen-warping effects going off every turn), the build diversity, and delving too greedily and too deeply into the caves.
Cogmind - haven’t played this one, but it’s on a list. You’re a robot. You’re building yourself from parts as you go, fighting other robots and stealing their parts.
CDDA - one of my faves, but definitely not something I’d recommend as an intro to this genre. You’re a survivor in a zombie apocalypse. Go do things and don’t get bitten. It’s a sandbox - survive as long as you can, achieve a self-set goal. The distinguishing feature of CDDA is how realistic it tries to be - crafting is very complex, you need to track your thirst, nutrition, and sleep, you can easily get sick or get your arm broken, the zombies can track you by sight, noise, and lingering scent… My favourite part is surviving long enough to build elaborate apocalypse death mobiles, Mad Max style.
The real juice of modded minecraft is in the modpacks - curated sets of mods that were configured to work well with each other, frequently with some custom recipes added by the pack developer, and sometimes some kind of a quest line to guide you through the pack and provide a more structured experience. There are many different types of modpacks - kitchen sinks (large collections of mods, frequently without a lot of balance tweaks or changes, for a more sandbox experience), questing packs (with the aforementioned quest books to guide you through the mods), vanilla+ packs that intend to expand on the vanilla minecraft experience and not change the gameplay loop significantly, packs focused exclusively on magic or technology mods (or both), expert packs (questing packs with heavily reworked recipes, where you need to build elaborate machines and automate stuff Factorio-style)…
I’m not up to date with the modpack scene, so can’t really make you a definitive list - back on reddit (sigh) there is a r/feedthebeast community that specializes in modded play.
That said:
You’ll also need a launcher to install these packs - FTB have their own if you want FTB Academy, otherwise there are some options such as Curseforge (do not recommend, eats resources just by existing), Prism (seems to come up a lot as a recommendation), or GDLauncher (what I’m using).