This was an interesting read! I definitely felt the effort and care you put into this

Since you mentioned you were looking for constructive feedback, here's some:

Overall, it seems like an interesting system since the numerical values of the dice aren't the centerpiece of the srd, which is fairly unique. I think this could have been communicated a bit more strongly even because most other systems do focus on the numerics, so I was splitting my attention between the mathematics of expected values and double roll chances, when only the former was really needed for most sections. Just a blunt opener like "This srd is built on the chances of rolling doubles of any value with nd8 but rules for taking values into consideration do exist." would be my instinct there.

The only part of the srd that really left me confused was the table page 4, or more specifically, what the top row of the table is. As far as I could gather, the double to quintuple rows are the probabilities of rolling that many identical d8 from among 2 to 5 d8. I think the rop row are the chances of getting only unique values but the wording "only single" is very ambiguous and I definitely didn't get it on my first, second, or third read.

Oh and this might be intentional but the watermark in the background of all pages is really difficult to decipher since light blue and light grey are so similar.

Overall though, this still seems well-crafted and I'd love to see different people do something with this.

I also really like how you expend a few pages going over the very basics, that just makes the pdf very accessible.

Concluding with a small example section definitely helped demonstrate how the Diamond Doubles srd can be applied, so that was really appreciated :3

Cool use of d8's! It's nice how versatile it is