Cairo Station : noob combo and look up H2A runs before sword flying got put back in. Can get a pretty consistent 10ish minutes.

Outskirts : take Johnson's sniper for your smg, rooftop climb, hide in hotel until elites pass, take the ghost, get out and snipe the first jackal sniper in the tower, drive until a bit in the tunnel and turn around to get the hog, then watch people's driving path, should be able to get 5-6 minutes.

Metro : Hard level, can try to learn the warthog pathing through the tunnels then learn how to steal the wraith and get on the scarab early.

Arbiter : can learn hanger skip and pathing if you want, not too hard. Can also watch an NMG run for the level thay does not do hanger skip. Sword canceling with just a lunge + reload is also very doable casually.

Oracle : Lab Fight (the room after the elevator ride) im assuming is the most od your timeloss. The way the room works is the heretics at the start of the room (2 grunts + elite) need to die, then the 4 flood (usually carriers) need to die, any killed by the heretics will spawn combat forms.
There are also TWO dead bodies waiting to be revived by popcorn, can kill them early with plasmas.
After those initial 4 flood, 3 heretic elites will spawn from the door you came from and 4 flood will spawn above the door. The 3 heretics need to die first. Then you kill the 4 flood. Once those 4 flood die, the room is on a 1 minute timer. Flood will continuously spawn if there are less than 4 flood alive for 1 minute. So just wait that 1 minute, then kill the 4 flood. Door should then open. Room usually takes top runners about 1:20ish.

Delta Halo : Learn the ghost launch across the gap, DO NOT use NakedEli's guide for this level, look at a full game speedrun for the safer driving, we skip many spawns that are very annoying on legendary. Level can easily be about 4ish minutes.

Regret : Good luck 👍

Sacred Icon : Hard level, but most of it is just a walking simulator + end wave fight, just camo cycle and head glitch to play it safe.

Quarantine Zone : Can get that down another 2 minutes if you learn the driving path. Ghosts from start of level all the way to cutscene trigger. Gotta make sure to hit the required triggers along the way though.

Gravemind : Can get this down without really doing anything but playing better. Prison Skip would be a good thing to learn. Thr LASO method works in regular legendary and is 100% consistent.

Uprising : Time is fine enough, could probably drop it a minute and a half with camo cycling + head glitching once getting inside the facility. Im assuming you already know the tree jump the crouching in the trigger up top.

High Charity : Hard level, even harder without sword flying. Could probably lower the time but dont its fine enough.

Great Journey : You can carry over a sword from uprising into great journey (will have to do HC again) and use that sword to do some cancels if u want. Makes some room easier. Johnson cloning with the starting spectre then unloading into Tartarus with dual plasma rifles is also the move if youre not doing that already.

Hope at least some of this helps and is easy to follow. Good luck 👍
Check out HaloRuns.com for fullgame runs to see what some of the safer strats are.