So, fun fact on that point, it's all about the degree! No, genuinely, while this is absolutely terrible and I'm not trying to minimize the issues you have (I myself have no intention to pass my genes on for different reasons), this really is all about the removal degree of the incest.
You need an, at least, fifth to sixth degree of removal for incest to be considered safe in any form (the term then becomes outbreeding instead of inbreeding). This degree works both vertically and laterally. From what I could find, for exemple, while Tay-Sachs disease risks are indeed heightened by inbreeding, the fact that in this case we go into outbreeding territory would make this safer in regards to this specific risk.
IIRC, there even were studies on that matter, and they found out that children born in this range even tended to be more healthy than average, but genuinely don't take my word for it, it's been a few years since I came across this information, and I'm currently too lazy to look it up. It's midnight and I have work tomorrow.
Thing is, it compounds.
Ethnic Ashkenazim are all at least 5th cousins. You NEED to get a DNA screening before marriage if both are ethnic Jews.
Rav Abby Stein has said her family tree is just as tangled as European royals! Descended from one specific ranbi "many times over".
You can't go down this road. Once you know, you have to avoid as much as possible.
Cousins breed once? Statistically unlikely to have significant problems.
Normalise marriage between cousins and you risk things going sideways in just a couple generations.
A population can quickly reduce in genetic diversity, and the humN gene pool is already relatively small (many bottlenecks over the past 10k years).
People should shop for DNA in wildly different ethnic groups, to spread the genes around.
Marry whoever you want, but go get ateast half the DNA somewhere else.
Or one could travel to places one's ancestors are not from.
On a personal note:
I'm not saying Jews should break nearly 2k years of tradition and proselytise again. I'm saying we need to be as welcoming as possible to people who want official Jew status.
Sure, hold them to the full standards, but be as encouraging and helpful as possible! Make them into the best Jews in the community.
Pretty good ethnic diversity at my home shul. Unfortunately, there's also a pretty strong Zionist bent there -_-, but that's not relevant. Just got remembered and got sad.
Sorry for the typos, I don't want to get out of bed to get my glasses.