I wouldn't say I particularly worry about it, but I do sometimes have similar thoughts.
I'm not diagnosed but pretty much fit autistic and ADHD criteria. I'm in my mid-50s, don't have a partner or children, haven't worked for some years so don't have colleagues. I have two siblings and a couple of friends, all of whom are a lot older than me, so it's likely they may not be around when I go.
Because of being ND and owing to certain life experiences, my natural inclination is towards solitary interests and activities. So, yes, I'm pretty much alone. It's possible, of course, that I'll make connections with people in future, but it's going to be very difficult.
OP, aside from your child and other family that you mentioned, I'm sure there will be others who will miss you. There are a number of people I knew who have died over the years, who were only really acquaintances, but I was upset at their passing and often think of them. If they were here I'm sure most of them would be very surprised to know that, but it's true.
I haven't been to many funerals but, for what it's worth, those with just a few mourners were often the most touching and I don't mean because of the low numbers. It's just my view, but many of the people in the larger congregations I've been a part of seem to just be there as "hangers-on". Whereas I'd probably only attend a funeral for someone I was genuinely close to.
It's easier said than done, I know, but try to concentrate on the fact that you do have people who love you and who will mourn you, and perhaps there are also others who you'd be surprised to know also think highly of you and would miss you.