Warning: long post incoming!
I have one autistic child, nearly secondary age. Low support needs, high masking, hyperlexic, high IQ, able to attend school (but only just). He has ARFID and PDA and can’t tie his shoes or dress himself or wipe his own bum or sleep in his own bed or be left in the care of anyone but me and DH.
Things are hard. Really, really fucking hard, a lot of the time. Particularly as he gets bigger and stronger - it can be scary when he lashes out. My life would not be manageable at all if I had a younger child to take care of at the same time.
YMMV of course - every child is different, every family is different. I do know larger ND families where things are just fine - but they generally all have low support needs & decent adaptive function. The sort of cluster of ASD traits where maybe you don’t like crowds or wool jumpers or loud noises but you are uncommonly emotionally stable and can manage perfectly well as long as you’re left alone to enjoy a quiet orderly life with very few other people around.
Having plenty of money and support is certainly a lot better than having neither - but so much depends on what support the child will tolerate, or can be helped by.
And never underestimate how much it costs to raise a child with even a ‘mild’ disability, starting with the almost inevitable, in my experience, loss of one parental income as the child’s needs become more complex. You can’t assume things will get easier when they go to school, or when they become teenagers, like parents of NT kids can. They don’t need you less - their needs often become much more complicated and expensive and mentally and logistically taxing, and involve so much exhausting high-stakes decision-making.
Most families I know with any autistic children have all autistic children - or at least all ND - often with needs that are in direct opposition to one another, and being the main caregiver in that situation is way more than a full-time job, with no respite and no meaningful support (and often a heap of judgement from teachers and professionals, and sometimes your own friends and family).
I went through a broody phase when DC was young, and I am glad I was already too old for more babies by that point. A lot of it was just hormonal, biological broodiness, but I think I was also hoping that if I had another child it might be a redemptive experience where I got to feel (and be treated by others) like an effective parent and a competent human being, able to comfort my child in the usual ways most children can be comforted, and teach them basic life skills and manners in the usual ways and on the usual timescales. And I wouldn’t have to feel like such a puzzling failure all the time.
And in a way, sticking with one has meant that I have had just enough headspace to work through all of that and toughen up and believe in my own competence and accept the reality of our lives, and find a sense of humour about it all - rather than dumping a lot of my own unprocessed shame on my autistic DC in an attempt to avoid having to feel it and sort it out.
That’s just my experience. And I’m not trying to be a downer. But sticking with one has worked best for our family.