While the SEC is a powerful conference, primarily in football, they are still a regional group of universities that, for the most part, are not on the same level as ND, the B1G and others (Vandy being an obvious exception).
Thus, ND is not going to the SEC. ND will preserve the current structure of the P5, outside of a few fridge adds/subtracts, for as long as possible as it allows them to remain independent, which is ND’s primary goal, especially their powerful alumni base.
ND views itself as a truly ‘national’ university with no peer in the regionally based conferences. Plus, they have some bad blood with their most natural partner, the B1G, due to some historical religious issues (not so historical to Ohio St) and more recently when ND flipped-off the B1G to join as an associate member of the new B1G hockey league. They only joined the ACC because the ACC offered a juiced-up Big E sweetheart deal to ND and helped their non-football sports.
The only way that ND will join any conference is if UNC or Texas bail on their current conferences, which would result in the decimation of the ACC or XII respectively and create a P4 with 18 to 24 schools each. In that scenario, ND would likely be cut-off from a football championship game (4 team conference oriented playoff). Thus, ND would join the ACC if the XII died or the B1G if the ACC died.