My advice is to ignore the official labels of "unit, ss, mainstream" etc and take a really good look at the actual on the ground provision for children with needs as similar to your specific child as you can find.
My sibling and one of my closest friends both attended special schools growing up - no employer has ever blinked twice. My sibling is now a graduate and my mate has a list as long as your arm of professional qualifications. Both are able to work and lead independent lives etc, etc. Neither would have got there without access to specialist intensive teaching during their childhoods, instead they'd have been consigned to the scrap heap
. Mainstream would NOT have meant they'd have had 1/4 the quality of adult life they've been able to enjoy.
They are both "mainstream adults" for want of a better way of putting it, and I suspect that underneath the fear of the special school label that's the long term outcome your partner desperately wants for his child. (I think this is probably the angle you need to go at him with).
There are amazing mainstreams, and ss's out there and diabollically appalling examples of both. Ofstead is irrelevant as far as judging educational provision for SEN, my own child was abused in a school considered outstanding by Oftsead yet is positively thriving in an environment these so called "experts" consider merely satisfactory. Sadly Ofstead is now about politics & the latest ministers faddish pet project, rather than the the real evidence based science of learning imho.
My own child is now doing really well in a mainstream with a special unit. Thing is the unit is for deaf children, NOT ASD like my son is. It wouldn't have been an obvious first choice iyswim, yet it's the best for my child due to the specific ways ASD has affected him in particular, and the teaching styles and knowledge & sheer dedication of the staff at his current school.
ASD is such a massive spectrum, and the sensory issues associated with it alone can mean that the perfect environment for one child may be hell on earth for another. Look, look, and look again for yourself. Examine with a fine tooth comb, ask pertinent searching questions. If your partner is unwilling or unable to join you in your quest then you'll have to put your child first.
Also bear in mind that children change as they get older - be mentally flexible enough to consider that your child may switch between special school and mainstream over the years as she develops. A helluva lot can happen between the ages of 4 & 18.