DC2 is like my little buddy and just wants to play with me and chat to me all the time, and I want more of that. More 'normal' parenting
I absolutely get this, OP.
I have 3 DC with ASD and other SN.
It's completely normal to feel like that. But I think it's worth considering that same statement above from DC2's perspective too. By having another sibling, even one without SN, the amount of 1:1 quality time she gets with you will inevitably decrease.
The huge burden the sibling of an SN child carries shouldn't be underestimated My middle DC, who, unlike the other two, is able to follow instructions and wait his turn, has had to grow up much faster than he should have and lives in a very unfair world (different behaviour expectations, days out he's looking forward to scuppered, etc).
He would absolutely love to receive more 'normal' parenting from me, but as he has the least time-dependent support needs, he's always the last one I get to.
I get why you want another child, OP, (and even I've occasionally mused to myself what it would be like to have the mini-me I was actually expecting to have!). But I would definitely look at this from both existing DC's perspective and think very carefully.
It's very easy to focus on whether the potential DC3 might have SN, and that's obviously a major factor. But the reality of having any extra human in mix here is a huge consideration for the whole family, and only you can know your situation.
The other thing to bear in mind is how both DC's needs might increase as they get older. Hormones can wreak havoc with ASD during the teen years (and my DDs weren't even diagnosed with ASD before puberty, partly because neurodivergence often presents very differently in girls).
Of course, you can't predict everything, and anything could happen years down the line (further diagnoses, MH issues (DC or parent), loss of support network, relationship breakup, etc).
Hopefully, whatever you decide, it will all work out. But you know more than anyone that life doesn't tend to pan out the way you expect!