I never actually converted manyuus, since the ratio is off.
Exp-wise the 20-year old one is worth two and a bit of the 5-year one, but you need to exchange it for three.
But that's maybe because I decided to level all of my girls, no matter the rarity.

At the point I'm at, I still convert equipment into Bond Crystals.
I also convert the lowest equipment seeds into the middle ones and recently the middle ones into the high ones, but that's only because I have Bond Crystals to spare (the exchange ratio for Bond Crystals is better for mid-level equips, since you get a lot more of them).

If I'm lacking second affection presents I also exchange any presents I have too much of for them.

Once you get to the point of evolving personal equipment for the girls, the conversion of forge spirits into the ones without standard affinity will be needed.