The Hill Sphere idea had potential, but doesn't deal well with extremes.
There's also the matter of SARP's weapon and sensor ranges being out of whack too.
As I see it, SARP has 3 propulsion requirements for its narrative:
1. In-combat speed. Which is, narratively, knife-fight thruster propelled fights like you see in most action-oriented sci-fi.
This is mostly a level to retain a series of contradictory requirements. You need to have various weapon types, from slower railguns, guided warheads and instant-firing beams be relevant - especially on delay-to-impact considerations. You need to have speeds that allow a degree of orbital travel which being able to navigate through obstacles in space fast enough to make them relevant to the narrative. You need to be able to interact with various units sizes from power armor, fightercraft to starships.
2. In-system travel speed. Which is, basically, the speed you travel at which needs to be notably higher than the low speeds you need to support knife-fights in space.
This is the speed at which ship need to travel when not in combat, but not warping around. SARP has required warships to do jumps at this speed as well (I call them tactical jumps, Wes calls them microfold jumps - the truth is, with the distances involved, it's all sublight travel). This is mostly a deal of finding excuses for ships not to constantly be traveling at that speed in order to make them vulnerable enough to motivate the close knife-fights without a plethora of superluminal maneuvers.
The Hill Sphere may be an imperfect solution (it deals badly with extremes; i.e.: Earth vs Jupiter), but its base notion had some merit. Fights in outer space would be a spagheti FTL-maneuver making efficient weapon use shown by our narrative far-fetched without going back to the FTL torpedo (laser+1). But then, there's little reason to fight out in outer space because there's nothing out there. This makes the one thing of value in the universe, resource nodes like asteroids/planetoids/planets carry a degree of greater importance. Hence, it's useless to leave a force into empty space, so you'll cluster them around points you need to defend (the Nepleslians understood this sooner than Yamatai did). So, in implying that a ship is affected by a Hill Sphere and forced to slow down, we're also applying a consentual - if non-verbalized - notion that "if we go there, we have to slow to sublight, and we become vulnerable to attack).
The notion of 'sublight jumps', to me, comes with a caveat. Since we're all based on mass and gravity considerations, in my opinion briefly countering it required compensation that carried a tactical price - a risk - and that was dropping the shielding (which is treated - in majority - as gravimetric) in order to squeeze out that brief jump.
There's a way of 'interdicting' movement, even those brief jumps - and what replaced the interdiction systems we used a couple of years ago (very inconsistently) is simple applications of graviton beams on the target (which do work, anyways, if the target is unshielded). Or otherwise cause direct damage on the vessel/use specialized ammunition (subspace detonation; brief, but can ruin someone's day).
3. Intra-stellar travel. That's the hyperspace fold drive. Regardless of how fast it goes, it's your means of travelling from one star system to another.
Fold implies a similar weakness to sublight travel. Be close enough to a big center of mass, and you won't be able to manage to charge a fold jump (or it'll take more time/more power). Except fold is much pickier than sublight is. Interdiction is the same; apply a graviton beam on the target.
...
From what I can see. Wes doesn't fancy travelling in space in the way of making long sea voyages. Little to do, not a good vehicle for narrative. Slowing ships down in star systems however reverses that a bit. You're forced to look and interact with an environment that does have objects, where the layout of a star system does become more relevant. I've found that SARP, in the past, trivialized star systems a lot (planets are trivialized too - we could have most of the content in SARP actually be different cities on a single planet; or we could have different empires actually be different planets in a star system considering how most races are a spin on humans) and the Hill Sphere notion does comfort me with them becoming a bit more relevant simply due to most of the action happening at the periphery of every planet.
Maybe there's a better solution, but the considerations between points 1, 2 and 3 above need to be balanced out for that. That means needing to deal with a lot of stuff to have it balance against each other.
