1. Using set seeds (if random seed), splicing runs, going into creative, using commands like /give, /tp, etc.
2. I think Optifine messes with mob spawning and zooming isn't allowed. Just use sodium, lithium & phosphor as an alternative for performance enhancement.
3. You can't use snapshots or pre-releases for speedruns, since some have weird bugs that can be exploited.
4. Hypermodern is an easy (sort of) strategy that involves going into oceans to loot shipwrecks, treasure, monuments for iron and gold. Bastions are also used to obtain almost guaranteed pearls, obsidian for blind travel, strings for beds, and fire res for an easier blaze fight. I recommend watching Couriway's video titled "20 MINECRAFT SPEEDRUNNING TIPS FOR ANY PLAYER!".

Minecraft: Java Edition

A few speedrunning questions

the_xdude

benchbotch

unseaal

Creepet

Minecraft: Java Edition Category Extensions

!findseed the map

Minecraft: Datapack & Modded CE

Filtered Seed Glitchless

AeroGelo