I discovered that I could get the "tough as nails" mod running in mc 1.21.8 so I tried a survival game. Hard, but fun. Not as crazy as the RLCraft modpack. TaN just adds thirst, body temperature, and hit/damage location. No new mobs like dragons or sandworms, and no RPG-like game extensions. Thirst works like hunger in vanilla. Temperature means you can get hypothermia in cold biomes and hyperthermia in hot ones. If it gets bad enough then you'll start to take damage. Hit location means you take more damage from head and body hits. There's also things like movement modifiers.. if you jump/fall from too high then you can break your leg(s) and can't move as fast for instance.
I spawned in the middle of a desert surrounded by savanna-ish plains. No water, and I immediately started to overheat. Went underground at the first opportunity and waited for night so I could sneak out and break a couple trees. I'm also using the Lithosphere and Still Life mods however so the biomes tend to be bigger and going anywhere near the forest at night is dangerous. At least mobs are easy to see at night in the desert. I grabbed a couple trees and some dirt from the very edge of the forest, then took it back to my hole in the desert. I put down the dirt, planted a couple saplings, and managed to get a good start on an underground base. (In spite of a couple zombies and a spider or two.)
I found that digging down lets you get away from the heat. In fact if I dig down too deep it starts getting too cold. Guess it'll be a while before I can dig down and get any diamonds. (Standing near a running furnace does heat you up though so that might be a way to do a little deep digging if I can get enough trees and coal for torches and to keep the furnace(s) running.) Water is an issue but so far I've been able to drink rainwater. There's a tiny bit of water on the ground nearby too. Drinking directly from pools can get you sick with this mod but swimming in it does let you cool off. I found a bit of iron while digging.. not enough to make iron gear but enough to make a bucket so I took some water back and built a small underground farm, then covered it with glass. I also found a good amount of copper, from which I made a canteen. Haven't figured out how to purify water and get it into the canteen yet tho.
Still haven't found any sheep for a bed either so phantoms at night are a problem, especially since night is the only time I can really work on the surface without overheating.