I think because of the scene where he’s sitting overlooking a canyon, we’re meant to infer that he made a few stops to take a break, breathe, and probably sulk a bit. We know Viltrumites are incredibly hardy, so they probably don’t need as much oxygen as humans do, so planets with even lower levels of O2 would be enough for Nolan to replenish his oxygen supply. To be fair though, he is absolutely MFTL in open space, so it’s possible he did just pull the world’s most epic and sad “there is only one trip”, but I lean more towards my take.

Also, and I say this as someone who has contemplated suicide in the past(I’m much MUCH better now, let’s not get into it), it looks like he didn’t commit to the suicide until he was staring at the black hole for a bit. My head canon is that he was bouncing around, planet to planet, galaxy to galaxy, just letting his thoughts eat him alive until he got to that black hole. Now, looking at something that represents pure destruction, ruminating on how he fucked up and how destroyed the life he loved is, he finally saw a clean out, and decided to just let the black hole’s gravity do it’s thing…until he saw a chance to do something good. Something he wanted to do in his heart of hearts. Something he feels bad for not doing more of, and jumped at the chance to try to redeem himself, no matter how small.