Design changed again, to better replicate a high availability pro DC set up.
I forgot I had an RB5009 next to the existing one. It's now alive, peered with everything via BGP and doing what it should be. It's now an edge router: it advertises that is has a route to the Internet available though it's not as good as the other guy. Doesn't even use its own Internet link for own traffic: goes across the slipstream to the primary.
The previous 5009 sits waiting for VRRP to send things its way as the 2116 isn't working then will use a 2.5G port to talk on the 222 train and know whether to send across the switching plane to the sexay server or to send to its buddy 5009.
I ran out of 10G ports so, rescued from eBay, a CRS309-1G-8S-IN is ready to rumble and has been able to offload everything bar a couple of clients running GbE from the t existing switch.
If I'm honest that switch's lifespan isn't looking too promising. It's going to be replaced by some cheap unmanaged gigabit switch.