I was on a post baby career break when DS regressed with autism and have never got back on the career ladder but now work PT. I do regret that I have lost my salary and am unlikely to ever resume my career and do worry what it means for the future (pension of virtually zero, not able to help other children through university). However I do know it was what DS needed as he was left with inadequate support in nursery and leaving him there all day was just not an option.
Before you take this step I would think about whether there are other ways round it
e.g. are there any specialist schools your child could go to where needs would be catered for better during the day and not have the spill over into home - you would probably need to apply for a statement of SEN?
Could you afford private support / training for you and partner to better manage behaviour at home?
Is there a behaviour support team e.g. linked to CAMHS or Local Authority you might be able to access?
Also think if you could cope on one salary.
Are you claiming everything e.g. DLA, direct payments you may be entitled to and might allow you to buy in some help in the evenings?
Have you worked out what you would get in tax credits etc if you were to give up work? It might be more than you think.
Would any of your children do equally well in a mainstream school? Could move to a good State 6th form? Get a scholarship?
2.5 years behind might be what is developmentally right for her, she may get there in the end but just take longer. I would probably be tempted to put some money into getting some behaviour input / training / support for you as parents (this should be available on NHS etc but often is hard to access) even if it means not doing holiday support and see if that improves things.
We do applied behaviour analysis (used mostly but not exclusively for children with autism) and once you have been taught strategies to manage behaviour you alter your parenting and get much better at keeping things in control. I am not saying its easy to stay on top of behaviour but now we have the skills and techniques my DS behaviour is much better and outbursts short-lived.We also work closely with those who support him at school to ensure we are all being consistent in how we respond to behaviour issues.
You are also entitled to (unpaid) parental leave for a disabled child which you maybe would want to think about to give you all some breathing / thinking space or time to go on training etc
Giving up work will not in itself be a solution if you do not know how to get control of behaviour.
I know someone who puts their autistic child in nursery when they are not at work as they cannot cope with their child's behaviour at home. Having the skills is just as important as having the time.
I would also say that State mainstream was a disaster for my child and I know few parents with children with SpLD who are happy with the help their child gets so I would be wary of giving up an income that funds good quality private support. But you need to regain control on the home front as behaviours could become entrenched.