I think they spawn in groups.
I dunno about chicken jockeys but you can keep babies behind the wall by making the slot less than 1-high with some trapdoors (preferably iron.)
Well it's "only" 76 double chests full of dirt.
So get a diamond shovel with Efficiency IV and Unbreaking III, breaking dirt at 20 blocks per second.
Not accounting for time spent picking up, transporting, and storing the dirt, that's "only" 3.6 hours breaking dirt.
I would do a 128 x 128 area first, to see how you like it.
If you have diamond armor with Protection IV and some healing/regeneration of some sort, then just dig a cave with a low ceiling and spawn the Wither in there and whack it to death with a sword (with either Sharpness or Smite at a high level.)
Quaff an 8-minute potion of Night Vision beforehand because all your torches will be blown away.
Parts of your cave roof and maybe floor will be blown away too, of course - but the Wither will still remain within sword-reach. Most of the Wither blasts can't even destroy cobblestone. No need for an obsidian cave.
You can make it even easier by making a long thin corridor leading to the Wither cave-room and then nail it with a bow as it blasts its way to you, saving you from any damage to yourself until halfway through, when you have to get to work with a sword.
At any rate, dealing with the Wither in the open air is a bit of a nightmare IMO!
Mine some iron if you haven't, make some shears, shear a couple mooshroom cows and they turn into normal cows, which you could then breed with wheat. This allows you to get steaks, one of the best food sources in the game.
Jungles do have chickens and melons. You could push a chicken into the water and then try to pop a boat on top of it, then you have a chicken in a boat. Or as noted above collect eggs, which is slow from random chickens unless you get a chicken farm going in the jungle (that might be the most straightforward way.)
Is that render distance bug still in play?
If so, then the view distance on your realm needs to be >= 10 (if that's settable.)
A little bit below 10 is not too bad for mob spawning rate, but well below 10 will throttle it way down.
Drowned can spawn in mid-ocean (not necessarily on the ocean floor) but they will not spawn in land biomes. Only in ocean/river/swamp and other watery biomes.
If you were building under the ocean, you could still prevent them from spawning in your waterways by keeping the light level up.
Also a personal peeve. When I chop a tree, I generally even wave my hand at all the foliage to kill it also, especially near my base or in a forest I've planted for use. Too darn impatient to wait around for it to decay naturally, and I generally want the saplings to replant immediately.
If you're cutting down a bunch of trees, the first one will have pretty well decayed by the time you're done the last one.
But how to replant? I just keep a small chest nearby with extra saplings, which I can use to replant trees while chopping. Once done with all the trees, chopping and replanting, there will be a new load of saplings to store in the chest.
I can try standing in water while I is mining around lava to avoid more deaths by lava, the only cons of using that method is that I can't use my shield in case of a creeper coming from the back and ssSSSssSSSsSing and exploding on me. Should I have used water while I fell into the lava pool? In the video I tried to run over the lava block but it did not work and I fell into the lava and died ☠. ...
Well it would go like this:
1 - Place a block of water at the same level you want to dig away.
2 - Do all your digging at that level, starting at the side of the water block. Any exposed lava blocks will be frozen into obsidian.
(Why couldn't you use your shield while standing in the water? Is that a thing?)
3 - When you have removed all the blocks at that level you want to dig away, dig a hole and place your water one level further down.
Have a pail of water on your hotbar and your first thought when fallen in lava should to be place that water bucket ... next to you, at your feet, wherever - since water flows, almost anywhere is good. I would choose to plop it against the wall I am facing, probably. The water will also prevent you from burning to death with the after effects of lava.
Well don't step down into lava would be the basic rule!
If you find yourself standing in lava, with your head free, then placing water almost anywhere (displacing the current block of lava for example, or having the water flow onto your lava) would do.
Stepping out of the lava would be a good first step.
But to really answer the question, If you're digging around lava, do it while standing in water, and do it one layer at a time. Seems like you were taking the right approach (digging around the diamonds) but you didn't have water at your feet and you didn't clear one layer at a time.
Might as well finish all the digging before getting the diamonds too. One thing at a time.
In reasonably flat ground, you can take care of night-time monster spawns aboveground by placing a torch at all coordinates where X,Z are (..0 , ..0) or (..5 , ..5)
You're placing torches every 10th space North-South and East-West.
Then you're making the same pattern offset by 5.
This also makes a pretty diamond pattern of light when seen from above. It is also easy to do if you are watching your coordinates in F3, the debug view.
This is enough light to prevent spawns on any surface as long as the elevation change is no greater than 1 between a given block and any torch.
If the elevation change is a little too high, there will be much diminished spawns, but not zero. (A light level of 7 will spawn mobs, but at 1/7 the rate of a fully dark block - and most blocks will still be lighted at 8 or above.)
Not all trees need to be cut down, because mobs cannot spawn on leaf blocks. A tree trunk in line with your torches could darken some blocks enough to make spawning spots. Things that are not in line with your torches (not at coordinates ending in 0 or 5) are reachable in multiple ways by light and so can be ignored. Acacia trees can have spawnable blocks (logs) in the air, so need extra light in the tree.
Back when I looked into swamp biome slime farms the ideal circumstance would have been to use a 'standard' in a slime chunk that was also swamp with the darkroom extension (layers 50-70) acting as an occasional booster…
The difficulty with this approach is that the darkroom needed between 50-70 spawns other mobs which requires active mob removal (to avoid filling the cap with static mobs) & complicates the transport/harvest phases unless one either built two farms using different mechanics or sorted the mobs before passing the slime to the bottom slime-only farm.
The new undead mobs sinking in water offers a comparatively simple way to remove these, but spiders are more difficult to sort. (With a carefully designed harvester/kill room creepers are not at issue as they can be killed withou allowing the line of sight needed for them to activate.)
My approach was just to have a mob farm that included slimes. The bigger slimes ground themselves against a cactus producing small slimes which would take the Long Fall (tm). Spiders likewise would grind against the cactus. Skeletons, creepers, zombies, etc would just drop into the middle and be sluiced into the fall zone.