The diary is a really good suggestion. Descriptions of the meltdowns written just after the fact will be really useful for any health care workers. Unusual feelings, how it started, possible triggers, mood beforehand, mood afterwards, tiredness level, hunger level,... Plus it will help you to see the patterns which are probably there.
If you have any parenting/relationship concerns, then as a teacher you know this already but praise the good, try to ignore the bad. The 'good' attention has to outnumber the bad by quite some way so they can start responding to the positive. One of mine was difficult a total little sod and I genuinely couldn't find good because he was so negative. So I had to contrive ways to offer praise. " DS, please could you be really helpful and help me put this bowl in the dishwasher - wow, thank you that is really helpful, well done"
(FWIW, he has a few ASD traits, and a few ADHD ones - functions OK but the concentration difficulties are still there)
Is it a fear response? In retrospect, DS maybe couldn't read facial expressions well, and got scared of me getting annoyed at him. It felt at the time like he went to 100% negative instantly but now I think he went to 'scared, can't cope, shout NO and scream'. And then I'd get annoyed... (Sorry to suggest it - but it never ocurred to me and I wish it had.)
Things which generally helped as he started school were moving bedtime 30 mins earlier. And making sure he didn't get hungry while tired - so a snack in the school playground. Hunger or tiredness and self control went out of the window.
You said your DD is tired at the moment - is a nap possible?
And do choices help at all? Getting dressed isn't negotiable (actually, we dressed multiple times in the playground because he had 100 % refused at home...) - but a choice whether it's this blue or that red t-shirt gives them some feeling of control over the situation. For my DS, feeling control was important.
Talk to the preschool - the seniors will have seen a wide range of 'normal' before. If there is something up, they are likely to spot the evidence (both my DC were sent for eye tests on the advice of their preschool - they were spot on)
Good luck, it sounds like you need a hug x