Much more of an issue than pH here is your TDS (total disolved solids) as a proxy for this you should check your GH and your KH. If it is indeed something about your water parameters, I would look to hardness.
And before switching to 100% ro and remineralizing, I would try a mix of ro and tap. However, there is no point in shelling out for an ro unit unless you know it helps. I went through this process for handling Altum angels and I did it in a 29 gal not a 10 where it is cheaper and easier.
Go down to your local supermarket and buy 10 gals of distilled water. I used to pat between 80 and 99 cents a gal. Then the next batch of eggs you pull dilute your water with 25% distilled when you set up the tank. Check the params before moving over the eggs.
It is important, if you are adding Methylene Blue, that you remove if from the water when you get wigglers.
With the new mix of 25-75 ro/tap, so all water changes with a similar mix (hence the extra jugs). See if that makes a difference. If not, you may want to consider trying it again at a 50/50 mix.
Also, bbs are pretty much worthless w/i 24 hours of hatching. You need to hatch out a batch a day. I used to ween mine off of live by beginning to mix frozen cyclop-eeze in with the live bbs at about 10 days. I would start with 20% of the feeding as the cyclops. Then I would increase the ratio of cyclops to bbs each day until at about day 15 I was at 100% non-live. I also removed uneaten food 15 minutes after each feeding. Baby angels sleep on the bottom at night so it is imperative it be clean when they do.
I agree with the suggestion about spawning pairs in their own tanks. You can use a 20 gal for this.
Good luck with it and keep the thread updates with what you try and if it worked or not.