We had a similarly awful transition to Juniors (altho Infants wasn't a walk in the park either!). DS (ASD / PDA profile) was put on a part time timetable almost immediately. He also had 1:1 from the start, but she was very junior (first ever placement).
I think you need a school who will work with you / listen and think creatively, and you often get a feeling whether they will do this or not (your DS sound similar to mine and bespoke and understanding is the key).
In hindsight the autism team should have been more involved with the transition and school were just very under prepared for someone like DS.
BUT, they were willing to work with me, they didn't exclude him (just did the part time timetable which whilst wasn't great gave DS time to adjust and me and school time to build our case and shout for support)
To give you some hope (hopefully), where we are now in year 4:
The SENCO is / was amazing and supported us in the EHCP application (which DS now has - I did have to appeal to get non assessment decision overturned, but that seems sadly v common).
The TA has been so quick and keen to learn - she has accepted all advice offered and now understands DS so well and is such a great advocate for him.
As part of his EHCP he does 2 days off site in an alternative provision (like a forest school), which he loves, and doing this enables him to cope better with the 3 days at school (still not quite full time, but we're working on it).
Our LA case worker has also been v good, plus the OT and autism team and they all genuinely work as a team and think creatively (the forest school thing has never been tried by our LA before but they were willing to give it a go)
I do recognise it was all only possible because I don't currently work so was available for a million pick up and drop offs a day (it felt like).
Everyone is also very forgiving of DS - he his own worst critic and after exploding is very remorseful - we all don't dwell and briskly say we'll try again tomorrow, then confer on email amongst ourselves on and triggers / patterns etc.
It is all quite Explosive Childy tbh (even tho I don't think school have read it).
Some staff are better than others with him, but the SENCO / HT recognise that and do tend to stop the 'old school' types working with him as it never goes well.
Sorry this is long (as usual for my posts).
In essence, finding a flexible, inclusive school can make all the difference. I think DS will prob go to a specialist secondary (maybe a ASD unit), but for now this is working ok.