My rotation is very similar. I see my #1 SB on a biweekly schedule. We've been doing this for nearly 2 years now. She's everything I could want in a SB: very reliable/dependable, very pretty and eager, DTF any way I want, very affordable, and for some unknown reason she seems to think I'm wonderful.
I'd love to have one more in my rotation to reliably fill in those alternating "empty weeks". In the recent past, I've had as many as 3 regulars on a set schedule. But, like you, for various reasons, that's not happening now. Some have disappeared or quit our arrangements, some have gotten too busy for a regular schedule, etc. So I am def looking for those BCD Targets of Opportunity to fill in those empty weeks, more due to the necessity of my current situation than for any intentional decision on my part to implement a new strategy.
But it's challenging! This week is a good case in point. Last week was an empty week, and try though I did, I was unable to arrange a date. There's one very hot, eager cutie I've known since 2018. She shows up every now and then and we always have a great time. Turns out she's available this week, so of course I said yes. But this week is also one where I'll be seeing my #1 SB, so 2 dates this week, none last week. And so far, nothing definite for next week.
If I find another very reliable SB I will try to schedule with her to visit each of those alternate weeks. That kind of dependability is very comfortable and very enjoyable for me. Meantime, I'll enjoy #1 and fill in the rest of the time as best as I can.