Will accepting Udom's quests on Groundbreaker lock me out of the best ending?

Solution 1:

Simply accepting Udom's quest does not affect your possible endings in any way.

Completing Udom's quest gets you to Byzantium early (instead of waiting for Phineas to give you a nav key post-Monarch) ... Udom will give you a nav key after you do some tasks for him. This also doesn't affect your possible endings. Of course, the Board isn't just going to trust any ordinary Joe off the street who claims to know where Phineas is, so there's a series of quests they'll give you to "prove" yourself.

Since you've not indicated what you consider "the best" ending, I can only say that if you continue down the chain of quests that are started by Udom, it takes a fair bit of time before you get to any actual decision point, where you need to decide if you want to side with Phineas or with the Board.

If you really want to side with Phineas after going the "snitch" route with Udom and don't want to figure it out for yourself:

You'll eventually be asked by the Board to put a tracking program onto Phineas's communications console on his secret asteroid base. When you go to the asteroid, talk to Phineas and 'fess up; he'll tell you to put the tracker onto his comm anyway and he'll hack it to send the Board mixed singals. (He'll explain that he expected them to find him eventually.) When you go back to the Board to report your success, they'll say that it appears Phineas has thwarted their attempt, but it doesn't reflect poorly on you because Welles is a genius. etc. etc.

Note that while Byzantium opens as a destination early, the side quests there are usually too high level for so early on.

If you actually clarified what you think the "best" ending is, I could be more specific in how things might affect it. But since you're only asking about accepting Udom's quest, the answer is a flat out "No" regardless. Just accepting the quest has no long term consequences.

(It is also entirely possible to completely ignore Udom's quest and play through the "naive" way. I did this my first play through. It is an entirely optional quest line.)