How to evaluate an ISP's peering / pick the "best" option?

My org has its own ASN and portable /24 from ARIN. Our HQ currently has two 1Gb circuits from two different providers (one is Spectrum, the other is a local regional provider that doesn't have much of a network outside our metro). We have 120+ remote sites that are all using Velocloud (SD-WAN). A decent chunk of those sites have Spectrum in at least one form (either coax or fiber DIA, sometimes both because there are no other alternatives as many sites are out in the middle of nowhere), so while we're not forced to use Spectrum, we do a lot of business with them and there are latency advantages.

The regional ISP has a relatively high cost and I'd already been considering replacing them. I was most recently being courted by Crown Castle. I don't know a ton about them, but I know they have a decent sized footprint as they own a lot of cell towers.

We are buying a different building and relocating HQ sometime this year. For various reasons, we prefer to continue to host in-house. Plus, the new HQ we are buying already has a massively overkill server room (raised floor, room-sized AC and UPS, generator, etc), the previous owner seemed to be using it as one of their own regional datacenters. The "new" building is already "on-net" with several ISPs; I don't have a full list yet but I know Crown Castle is one of them.

As previously mentioned, we will probably keep Spectrum as one of the ISPs, but I'm open to anything for the second pipe as long as I know the ISP is well-run and well-connected, and the price is competitive.

I just don't have any clue how to "evaluate" peering / connectivity, other than poking around in looking-glasses (which seem to be harder and harder to find these days) and running traceroutes to our most-used web apps. The main website we rely on is hosted out of Chicago but I don't know what datacenter or provider it is with. (We're slowly doing more with Azure / O365 but right now we just have a few toes in that water, Exchange is still 98% in-house.)

Is there any scenario where it makes more sense to get dark fiber / "private transit" from the building to a real datacenter, and then buy a single cross connect at that site to get "Internet"? This seems even more complicated since then I don't have a single entity to blame if I have problems with that "circuit".