I level multiple summoners per splinter, although most of my levelled summoners are from Beta and Chaos Legion. I didn't really level the untamed and dice summoners at those times, which I kinda should've though. Your favourite is definitely a good choice. A focus on a few specific splinters may also be great as it's easier to level just a few instead of everything. But I personally do wanna get all the Chaos Legion eventually maxed for silver, which does make it harder to also get the Riftwatchers cards as I still don't have any of those x/
I'm a bit torn between that too! So far I haven't used my hive power yet, because I do want to continue building it up.