Before considering the Bonvoy Brilliant card, let’s make sure we’re ruling out United, or at least how certain you are about flying JAL.
Award space on JAL opens up 360 days in advance, so if you want to book for next March/April/May you need to have the points in place now or within the next 2 months, because spring is a very popular time to visit Japan (and fly back to the US).
It seems you already have URs in place to transfer to United, 1:1, for one-way saver flights from NRT>LAX>SAN for 35k UA miles + $54.83 per person. United flies a Boeing 787-9 Dreamliner from NRT>LAX with a 3-3-3 seat configuration in economy. United opens up award availability about 337 days in advance. This might be a good backup.
As much as you may hate the idea of spending UR, isn’t things like this what the points are there for? And with you getting back under 5/24 in the next 6 months, URs should (hopefully still) be easy to replenish. Opening a Bonvoy Brilliant card will add a personal account to your 6/24, but I’m not sure how that’ll affect your plans to get back under 5/24.
One thing going for the Bonvoy Brilliant right now until 4/24/19 is it has a high offer of 100k Marriott points for $5k MSR in 3 months, after which the offer will probably drop back down to 75k Marriott points for $3k MSR. It does also carry an expensive $450 AF that’s not waived, but offset by the $300 Marriott credit and other benefits. Marriott points transfer to JAL at 3:1, so 110k Marriott (100k SUB and 10k from $5k MSR) will get you about 36k miles + 5k miles transfer bonus. (You need to transfer 60k Marriott to get a 5k miles bonus, so you’d need to have 120k Marriott to transfer in order to get those second 5k miles. Do you have another Marriott card to refer yourself to the Brilliant to get an extra 20k Marriott points bonus?)