I don't think you are being unreasonable for being upset about it, day to day life with a ND child is hard and I swear everything feels 10 times worse than it really is and I get that. But, you do need to have a really frank conversation about it all with your dad, you might find out more about how he feels and thinks about it and find things that work and develop on them.
When you have an ND child, you might find you have to explain things over and over again, to everyone.
Family, friends, teachers, professionals...it's draining and it is annoying.
Partly because of a lack of understanding around ND people, but sometimes people just miss understand or even forget especially pre-diagnosis and generation difference.
I have had to do many things I don't like doing because they need to be done to make life that little bit easier for him and for the family, so I do agree with the list idea and as someone said previously and Amazon List. I give my parents a list, they use that to get things appropriate for him and it works.
I don't know your dad, or you (obviously), but it takes time for people to understand and sometimes they never get it fully, even those you expect to understand as they are close to the child but I can't see your dad purposely buying slime to cause upset. It might have been a careless decision but also an element of denial (not necessarily 'buying for the GS he wishes he had') which is common and a diagnosis might help with that.
My son's dad (we are separated) was in denial massively when we started the diagnosis process and he still doesn't really get it but we have to watch our sons diet, and limit chocolate and sweets as these tend to heighten everything possible - Birthday and Christmas he comes home from his dad's with a TON of confectionary, it annoys me, it infuriates me in fact because I am the one who has to deal with the sugar overload on his return and the one who deals with meltdowns when I limit his intake, but I just remind that side of the family when I can, and they are slowly getting better and I definitely saw a difference when we got a diagnosis.
I hope the assessment process goes well for you and you are not waiting much longer.