Downloaded everything such as the mod menu, however when i slash with first person being enabled it zooms me out into third person and zooms me back in on first person

Better Combat does nothing like that, I recommend checking out what other mod is causing that.

Additional info that might help.

Better Combat discord FAQ:
Whenever I swing a weapon in First person, my head shows up and it blocks the screen fully. How to fix?
This kind of problem is typically caused by another mod that handles renderstates in a faulty way.
Many of these (formerly incompatible) mods have been fixed in the recent months. Please try updating all of the installed mods.
Usual suspects: Twilight Forest (fixed with update), Minecraft Comes Alive Reborn (issue still open)