It is a hard line to walk. You want safeguards but how much can you do before you’re the one actively inciting rebellion? The fact that there is an argument over whether Daeron contributed to Daemon’s decision to rebel by sending the Kingsguard to arrest him is a clear indication of the difficulty of finding a balance.
Unfortunately, I don’t think Daeron had many good options. Aegon IV’s machinations made even the simplest move a potential danger after he made Daemon an extremely potent rival so Daeron had to be very careful in how he acted towards him and the other Great Bastards. That flatout makes sending anyone to the FaithCitadelWall out of the question since that would be just the antagonist move that could inspire a rebellion. If it could be argued that the king was arbitrarily forcing his own kin into any of those orders “unprovoked”, it would be a rallying cry for all those who oppose him who would argue that the king is acting outside the feudal contract and perhaps even make claims of tyranny. Keep in mind that Daemon’s faction did use Daeron’s attempt to arrest Daemon as a cause for war in OTL and there is no reason that sending Aegor to one of these orders wouldn’t be used in the same way. Also, that could certainly be enough for Daemon to fear that Daeron would send him next to one of these orders in order to neutralize his claim which could end up what makes him rebel.
As for having Daemon’s children fostered at court, that is a politic approach that is not likely to antagonize Daemon, but it’s not without its downsides – it further advances Daemon’s social acceptability by having his line be court-raised same as Daeron’s own line, and puts them in a position that invites juxtaposing them against Daeron’s sons. Don’t forget that Baelor Breakspear’s Martell look attracted acrimony from those who resented the Dornish presence in court so I don’t imagine the presence of the presumably Valyrian-looking Blackfyre twins right there would do the royal family any favors. That said, fostering the twins is still be the most preventive but non-incendiary approach Daeron had despite the downsides. Daeron’s policy towards his
half-siblings was open-handed and he notably kept them close so fostering the twins would not look abnormal while it gives Daeron a useful leverage against Daemon and the chance to raise Daemon’s two immediate heirs to be loyal to their royal uncle and his heir.
That doesn’t make it impossible for Daemon to rebel mind you – the Blackfyre rebellion took place when the twin were 12 so Daemon could wait a few years till their fosterage ends or he could hope that the stigma against kinslaying, his brother’s peaceful nature,
and the affection he might have for his wards would stay Daeron’s hand from harming the twins. The fosterage doesn’t neutralize Aegor. Doesn’t remove Quentyn Ball or whatever resentment Daemon harbored. But it might make it harder to rebel, especially if his two immediate heirs don’t want to be a part of his cause which inherently damages his movement.