I currently start with a single mixed resin until the tds output rises to one, then I had a second Mixed resin. When tds output rises to one, I pull out old resin, moved second into first pass and add a second new mixed. Seems to work pretty good.
My TDS is about 160, which the RO reduces to 4-6 TDS, and DI reduces to zero for about 5-6 months. I used to get poorer performance, but adding a second carbon housing seems to have really helped in my case.
Perhaps try slowing your return pump. I agree with @Funlad3, isn’t the red pipe the emergency backup, for if or when the main drain is overwhelmed or clogged.
Sorry to hear this, why do you think the male flasher is dying? If the parameters are good, sounds like stress all the way around. Seems like you have plenty of room in a 75 gallon. I like your approach on changing water, it cannot hurt.
AdvancedAcrylics (John) might have what you're looking for. I have a couple of his and one is a narrow 4" x 24" x24" behind my tank. Works and looks great.