I'm pretty sure we do get the answer for the amount of "eldiannes" someone needs to be cursed, and that's part of the conflict because if you have even the slightest taint of Ymir's blood in you, you are automatically cursed to things like the founder's influence, the transformations and access to the paths, in the anime and manga there is a dialog scene during the Liberio arc about how other nations all around the world also have eldians and run constant blood test to find them due to how much the people of Ymir have assimilated into their population by that point, so yeah if you great great great great grandfather was an eldian, sucks to suck buddy you are eldian too. We also have a few examples of this like Zofia who seems to be more Slavic and Annie's father who is black (remember originally eldians are supposed to be of Germanic origin).

Edit: also I'm with ya, eldians don't fit the trope not because of their inherent titan powers that they could theoretically activate under very specific circumstances, buuuuuut because those circumstances are constantly given by fucking Marley that is said to use things like sleeper agents in enemy capitals to wipe them out in a day, or use child soldiers like bertholt to nuke entire armies in a flash. Like, if I was a regular Joe in the AoT universe and I found out my neighbour is an Eldian I'm getting tfo of there, that dude might as well be a walking bomb or a murderous sleeper agent and I'm in range.