I initially bought the 8" coarse DMT but found it not being coarse enough. Then I bought the 8" XX coarse and found it perfect for my needs. Used it for 9 years and still enough diamonds left for lapping. Unfortunately I find that the 8" is slightly shorter than the Shapton stones I have been lapping. It's mostly fine but for faster dishing stones it's just not big enough.
This is when I bought the 10" DMT lapping plate. It's expensive for sure but I think it's worth it. Buy good, buy once. Don't make the mistake of buying cheaper plates and ending up buying the expensive option anyway. Though using my 8" XX coarse for 9 years I got full value out of it. I can speak for the longevity of DMT plates too.
Don't worry about the XX coarse. I would buy an even more coarse plate if I could. You really want faster cutting speed for lapping. I work as a professional chef and sharpen tons of knives daily. I don't have time to lap my stones. XX coarse isn't even that coarse and the diamond plate will wear down overtime to a point where you lap almost perfectly smooth anyway. The coarser plates last much longer.