Hi all, I've recently upgraded the ISP to 1gb+ and the HeX s that has served me well for years is tapping out at about 800Mb/s.

So I'm about to pull the trigger on a RB5009UG+S+IN and cAP ax lunch combo box. With plans to run a 10gb/S backbone in the house with the 2.5 to the ISP.

For second guessing sake, am I making good call or a mahoosive mistake? Or am I just missing something I should be considering? Are there better hardware options?

I'm running a similar setup (RB5009+CAPax). I love the RB5009, but don´t expect to run 2,5G Internet + 10G to your NAS. 4-5G throughput is more realistic with some useful firewall, QoS etc,
Also if you are considering MT WIFI: it is currently improving, but you have to expect to spend many hours tinkering with it. That said, the CAPax is a good AP, but beware of new bugs each release.
For segmenting the internal network you would need an CCR2116 at least, which is a bit pricey. I went for an Opnsense box, where I had my NAS behind a firewall.
Update: to avoid confusion I am speaking from different networks here.

I was thinking the CRS310-8G+2S+IN for segmenting?

If you're asking about adding the CRS310 to the RB5009 to offload switching from the router, then yes, this is a fine alternative to the CCR2116 plan, saving money and getting much the same performance inside the LAN. It lets the switch switch and the router route.

If instead you were hoping that the CRS310 would serve in place of the RB5009 as the gateway router, not a chance.