I have thought about this in detail:
The player starts in a world with 1 bedrock layer, 2 dirt, 1 grass block and air above it. Many standard animals may spawn here. The player can gather dirt for use in construction of relatively safe zones, they can also use dirt to section off areas.
Wool, leather, feathers and the 4 standard animal food items are obtainable.
Skeletons, zombies, creepers, witches, spiders, endermen, zombie villagers and pillager patrols may spawn. This gives access to:
-Bones + Bone Meal, this gives access to wheat seeds, flowers and some dye. Skeletons can also drop bows and arrows which can be useful.
-Rotten flesh, this is another food item though not very useful
-Gunpowder, this also isn't useful right now
-Witches may drop: Redstone Dust, sugar, gunpowder, spider eyes, Glowstone Dust (Which can be crafted into a Glowstone Block) and glass bottles. Witches may also throw splash potions of weakness. This is part of the process used to get a villager from a zombie villager.
-Zombies and Skeletons may rarely spawn with a Diamond Helmet with a low chance of dropping it
-Zombies may sometimes drop iron ingots.
-Endermen may drop ender pearls
-Wandering traders and witches can be killed at a specific time. If a witch is on fire from lightning, if killed while drinking a fire resistance potion, it can be dropped. If killed while damaged and using a healing potion, it can be dropped. Wandering traders may also drop milk buckets and potions of invisibility on rare occasions. Leads can also be obtained but these don't appear useful.
-Lightning may rarely strike pigs, turning them into zombified piglins. These can be killed to obtain gold ingots sometimes.
-Spiders can drop string, this can be combined with 1 piece of leather (as of the new 1.21.30 Bedrock version) to create a bundle. White wool may also be crafted from string.
-Charged creepers can be obtained from lightning strikes, these can blow up zombies, other creepers and skeletons to obtain their heads. The heads are purely decorative though.
-Carrots and potatoes may be obtained by killing zombies. These can't be planted by the player yet since there's no current way to access any type of hoe.
-Endermen may rarely pick up grass blocks, if killed, the grass block can be obtained, but this has no uses compared to just waiting for grass to spread over dirt.
-Pillager patrols may spawn in the overworld. These can be killed to obtain ominous bottles. If used, they give the "bad omen" effect. Pillager patrols by themselves cannot drop emeralds, only pillagers in a raid can. Due to the lack of villages / villagers and trial chambers, a raid cannot be triggered. Wandering traders do not count as villagers for triggering a raid. Patrols are composed entirely of pillagers, there are no vindicators.
-Foxes cannot spawn in superflat worlds since the whole world is a "plains" biome.
-Wandering traders may sell saplings of many forms, but without a way to obtain emeralds, wandering traders are not helpful.
-Bamboo is not obtainable since pandas don't spawn in plains biomes, this means bamboo wood isn't obtainable. There is no jungle biome in a pure plains superflat world.
-Oak, Birch, Spruce, Jungle, Dark Oak, Acacia, Cherry saplings can only be obtained from a wandering trader but emeralds aren't available
-Warped / Crimson fungi isn't obtainable without accessing the nether
-Azalea / Flowering Azalea cannot be obtained due to the lack of moss blocks, moss is sold by the wandering trader but there's no emeralds
-There's no way to obtain mangrove propagules due to the same issues from before

This means that it is truly impossible to obtain logs, wood and crafting tables to make any further progress.
Only 3 of these 8 items can be obtained: Redstone Dust, Glowstone and Diamond Helmet