You're probing for an easy answer because you think one exists that'll be as easy as handling the commodity 100mil interconnect you've been playing with. But the fact is a minimal satisfactory answer resides in the 2nd level of nature's bowels where the simple conservative models of Ohm's law and the laws of Kirchhoff that you were taught in basic physics start to break down as a useful standalone tool and must be supplemented with more complex models to remain consistent with real world observations. I'm not even talking about Maxwell's equations in the raw either...we're still several orders removed, sandwiched in a domain where lossless transmission line theory and lumped-element modeling still hold. Still not ringing any bells? No worries...most practicing EEs are quite deaf to them after college as well.

If you insist on settling on an answer constrained to classical 1st-order models, then you're in for a treat: everything should just work! The guzzintos perfectly match the guzzoutas, first law of thermodynamics is demonstrably preserved, motherhood and apple pie, happy endings.

Except that's not how the real world works...and it almost certainly won't meet any pragmatic definition of commercially reliable at the limits you're wanting to probe at. But why trust what I have to say? You have an entirely capable Artix-7 dev board at your beck and call. And you have an oscope that'll generate eye diagrams all day to your heart's content. And the Pmod interconnect is so trivial that you could even hack together a throwaway DUT load...or why bother with that when you could use one of your existing Pmods as a real world DUT to characterize?

I thought I left enough breadcrumbs for the incidental reader to self-service on how deep down this infinite rabbit hole will be sufficient to grasp the magnitude of the questions being asked, and I thought my language was appropriately tailored. Maybe not. Here's a few more courtesy hints:

1. The first remark was to steer towards qualitatively thinking about the role of parasitic inductance and how this parameter will dominate the interconnect at higher frequencies as skin effect kicks on. It's worth noting that you'll be quickly sucking hind tit attempting to control impedance through a board-to-board interconnect of this design, which immediately kills reasonable prospects for a huge chunk of the application space that FPGAs are chosen as a viable host for right out of the gate!

2. The second remark was to kindly say that EEs who care about SI couldn't care less about the fundamental frequency of your digital signal when what matters in practice are the significant harmonics that said signal's edges are comprised of. The term significant here is roughly defined as at least 10%ish of the fundamental's magnitude. To get a feel for the landscape, derive the Fourier series expansion of (keeping things simple) an ideal square wave and interpretation of the rest should naturally follow.

3. The third remark was to establish a ballpark basis for independently sizing up when the convenience of simple 1st-order models start to break down and it becomes necessary as a matter of due diligence to kick it up a notch. Determine the permissible distance your signal needs to travel. Determine the wavelength of the highest significant harmonic of the signal to account for (assume in a vacuum to keep it simple, completely ignoring substrate permittivity). If the ratio of distance to wavelength exceeds 1%, consult with a competent electronics engineer if you objectively care about quality.

That's about the best approximate free answer this engineer is willing to extend. Need better precision? I'd recommend deferring to a billable scientist.

(P.S. only about as fun as trying to explain the merits of idempotency to my wife /s)