I think you'll be fine with that plan. Though it's hard to say exactly for the second DLC but you can use your judgement based on your level and how close you are to 50 vs how of the main story is left to complete.
Either way as long as you're aware of the level 50 thing you should be fine because once you hit 50, everything begins to give less XP so it becomes harder to push much past that. Getting into UVHM at level 51 is not a big deal and pushing a lot further than that is difficult unless you just don't know and keep grinding.
Also if you stick to just main DLC quests they don't give much XP relative to main story Quests but I do recommend doing the Tina DLC ones anyways because the whole DLC is great.
Also, the only downside of starting UVHM underleveled is being stuck with underleveled guns and UVHM being tough for newcomers. If you use slag and grab a level 50 Unkempt Harold from TVHM I don't think it will be too bad regardless of your level if you're willing to tough it out a little.