I just do it when i feel like it.
Whenever I get them
I take out 5 days worth of iso expiring and sell the 10k basics. And rankup whenever you want.
15 seems the golden number per class with the crucible type events. I have my priority 15 champs chosen per class and just rank those up. Until i get all 15, i partially rank up alternatives (7r2. 6r4) to ensure that the 15 per class is maintained
I grind arena for units so I have 180 R5 4*, 50 R5 5*, 50 R5 6* and 60 R3/R4 7*, another 50 R2 7*.
I mostly work to R2 my 7*. I still have another another 90 7* to R2.