Multiple PCB system vs mixed-signal PCB
There are (as always) pros and cons; much depends on just how much more expensive the laminate is for the RF section (I know of a laminate that can handle 6GHz quite well that has about a 25% premium over more standard material). It will also depend on the complexity of the layout(s) involved; using a separate board for the RF section might well reduce that.

There are one time costs (assuming you are going to make less than a few hundred - some things do need to be replaced).

Truly one time is PCB tooling (assuming you do not change the design - if so there will be a tooling cost for each iteration).

Solder stencils are good for at least a few hundred (and quite possibly more) runs on an automated line.

A word of caution; the largest source of problems in PCBs is (in my experience anyway) is connectors but that depends on the number of mating cycles - if you mate up connectors (and have a positive locking mechanism) then you may not run into that problem.

Could you benefit from multiple PCBs? Perhaps - only by evaluating the options (and talking to PCB houses about costs) will you actually know.

To the details.

Clock distribution; you could use a clock buffer (typical solution linked - there are a number of vendors in this space).

Connectors: My goto here is Samtec as they have a really great range of connectors in the high speed / signal integrity area (there are, of course, other manufacturers). I have successfully used their connectors in many high speed / high throughput systems; as you believe, there are definitely connectors available (from a number of manufacturers) that will meet your needs.

My personal opinion (and it is only that) would be to go for 2 boards (digital and power on one). It is perfectly possible to do a single board, but that might require more effort in design (keeping digital noise out of a very high speed analog signal is a bit of an art form). I went into that subject (as indeed have many others on this site) here.

There are also a number of excellent resources available online.

Make sure that you have a very solid low impedance ground for all the sections (so that the return currents are all referenced to the same potential) and do any filtering to the RF section on the power side (either with a very low noise regulator or a ferrite filter (The details of what to use are very application dependent).

So you might benefit from a multiple board solution (certainly it will help you separate the analog section from the digital noise although you will need to take care with the ADC routing).