Plants need a rest period. So running 18/6 is perfect till you are ready to flower. Then switch to 12/12. Personally I would
never run 24/0. But that is a personal preference.
Yeah, I run my seedling 18/6 and never use a humidity dome… that’s just me and I live in a low humidity state. I’d almost say too much water from the looks. At this stage I use a mister bottle and just spray around bu not on the seedling.