No doctor or expert. But I did follow the AZ story. I understood, that Covid can cause blood cloths. A vaccine triggers an immune reaction, in that case Covid, so a particularly strong immune reaction, maybe together with a preposition of developing blood clots, can be a reason. Not sure if we know already what causes these side effects with AZ an JJ.

From a risk perspective, I wouldn't restrict either vaccine. But since we can easily use Moderna and BiontechPfizer instead for the most at risk groups it doesn't really change things, does it?

Most immune reactions will cause blood thickening. If your arteries are already full of crap they will clot, this is why if you are at risk of blood clots you need to be careful even when you get the common cold.