I am really leaning towards saying fuck it and doing it anyhow. I think the amount of times you say someone's entire name is so few and far between that it doesn't matter.
We liked Anneliese for DD2 and were debating between a few middle names, one of which is my MIL's name (Elaine), meaning the e sounds kind of run together at the end of the first name/beginning of the second. We decided that you really don't use your MN much outside of birth announcements, graduation invitations, wedding invites, etc.
Just because the name is his first name doesn't mean he has to go by it. He can go by his middle name.
If you most definitely want to keep Lucas as the first name, go with Paul- especially if you don't like your other option. Whichever way you go, you will be honoring your father. (I understood that David was your dad's middle name.)
"Hello babies. Welcome to Earth. It's hot in the summer and cold in the winter. It's round and wet and crowded. On the outside, babies, you've got a hundred years here. There's only one rule that I know of, babies-"God damn it, you've got to be kind.”
I agree...honor your dad. Paul doesn't bother me in the slightest.
We wanted to a connection to my mom's side for both boys. We were able to use a middle name as DS1's middle name, but got stuck for DS2. We ended up giving him the middle name of the town where my mom grew up (also a boys' name). The connection is not obvious to most people, but it's obvious to the people who matter. If you decide not to use Paul, maybe you could come up with something that makes you think of him.
My first reaction was Lucas David as we use our kids' middle names a lot. On further thought, Paul is really fitting for your situation, and I would absolutely use Paul without hesitation. Lucas Paul is a great name.