I think you might be onto something. It seems to be that you have to have strong-moderately strong Otsutsuki chakra in order to even live with the seal. That's why Jigen was searching for someone that could. He knew he would eventually find someone(s) that have strong Otsutsuki chakra and can take the seal, like him. Boruto hardly has any side effect when he is given the seal. He doesn't die, pass out, or experience anything strange. Either that's because it was given by a main member of the Otsutsuki, or maybe it's because of his strong chakra of Otsutsuki origin combined with a strong body of Uzumaki origin that helped him out?
He only has side effects when he uses it... and the fact that he can use it so well (manga) already is probably due to what I just mentioned. He's a lot like Sasuke in this aspect. When Sasuke got the CM I mean. He had potent Uchiha chakra and a fairly strong body for someone that wasn't Senju or Uzumaki. He had side effects (because his body still wasn't strong enough), but was able to take it and control it damn well within hours of receiving it.
Meanwhile Kawaki I think struggled a bit with it if I recall, and a bunch of kids died before him too. Kawaki either is tied to the Otsutsuki by chakra too, or has an incredibly strong body to withstand the seal. No idea if he had to work hard to control it, or if it just came to him easily like with Boruto (once it activated). I assume he probably had to train with it and it may become a plot point in a bit. Like he might get real frustrated the seal comes easy to Boruto once he starts training with it.
Side note and slightly OT, since I did mention Jigen just once... and he seems to be just as involved in this seal bs than the Otsutsuki... What if there was a branch family to the branch family of Otsutsuki? What if it's revealed that Hamura (or some unknown full Otsutsuki) had a kid that wasn't exactly Hyuga or Otsutsuki on Earth? What if that's the clan Jigen was from and that is why he has the ability to give the seal? He's stronger and more pure than a Hyuga, but a lesser being in the eyes of Otsutsuki. Some weird hybrid in-between clans type of thing? I mean he doesn't look fully human, he has powers that have never been shown before, and he can give the seal to others... He has to be part of some clan closely related to the Otsutsuki.
