Is Feudal Elective better than Confederate Partition?

Solution 1:

If you've got a good heir and/or good relations with your vassals and/or lots of hooks to use I think going elective could be worth it.

Elective can be high risk/high reward, if you got good relations with your vassals they're more likely to vote for your candidate though it's far from a sure thing, if your candidate is pretty craptastic as can be the case with the eldest they might not vote for them regardless of how much bribes affection you send their way. You can however use a weak hook to force them to vote for your candidate for 10 years, you only need to blackmail convince a majority of the votes.

If you got none of these things Confederate might be a better bet, getting titles from your siblings is a pain but much less so then getting them from random strangers.

Solution 2:

Using Elective succession could potentially help you.

If a title has an election, you can see a list of all candidates and electors, and who has pledged their votes to whom. In your case, if you have good relations with the electors for the secondary Kingdom titles, you could end up keeping a title you stand to lose. If you know that you will lose the titles under confederate, it might be worth it to switch.