Hey,
Sorry to hear that you're having a tough time (and your daughter too!) Have you asked your GP for a referral to the peadiatrician?
If you make a list of your concerns & examples and take this to the GP they can refer you to one of the community peadiatrician who can put your daughter on the appropriate pathway. For my DS we first had genetics, bloods etc done before being assessed. The team can also refer you to local support groups, parenting courses (some are better than others!) And services locally.
For now there's no harm introducing supports for your daughter- visual timetables (you can make it yourself or download) with simple pictures of what is happening today. Ie: wake up > brush teeth > breakfast > swimming > lunch at home > toy time > dinner. I'd also recommend a now & next time table too so now: brush teeth next breakfast.
For playdates she might be better with 1:1, something structured and shorter and at home in her comfort zone with visual aids to help. So on the playdate they could do baking, followed by playing shops (shop keeper & customer) then drawing and a snack then home. (This might not work and it's a bit of teething trying to work out what works for your family!)
We found role playing a play date helped my dc alongside picture prompts / a social story to look at in the car- so we would talk about going to the park for example. You wave and say hello, then you show your friend what is in the bag that we brought, (I always bought a couple of cheap balls/ a foam aeroplane etc as a starting point) then it's time to build & play with our aeroplane, afterwards we will feed the ducks then it's time to go home. We take picture cards to remind my dc where we are in the playdate & to remind them when it'll be time to leave. If you can arrange it, if you're meeting at a park or somewhere busy it might be best to arrange it at either the very start of the day (9am) or after 3pm - these are usually the quieter points in the day and less visually, auditory stimulating and much easier to cope with (ontop of the social interactions!)
You can also get some sensory toys such as the chew buddies (necklaces and bracelets), ear defenders and squishy toys.
We found private Occupational therapy worth its weight in gold for my childrens escalating meltdowns - the OT gave us ideas of what helped & what made them more disregulated - my son for example is better with deep pressure it calms him whereas my other dc needs lots of spinning & movement.
I hope your dd has a calmer day tomorrow and you manage to get an appointment with your GP.