Working out some Superscience Space Drives

[Varyon]

So, I've been working on the star systems and "aliens" of my space opera setting, and I'm currently taking a look at the superscience spaceship drives. I already know how I want them to behave in combat - they roughly function like WWII fighters (for the fighters, obviously) or naval vessels (for the capital ships), to the extent of even having roughly comparable accelerations and top speeds. When not locked in combat, however, they need to travel faster - a lot faster. I want to scale things so that, theoretically, a capital ship with 3 drives would take 1 month to go from the center of the system (ignoring the fact there's a freaking star there, hence the "theoretically" bit) to the helioshock (termination shock, where the solar winds slows down to subsonic speeds), as the helioshock is where they can transition to hyperspace. I also want the drives to function as pseudovelocity boost drives. That's all fine and good... except it means such a ship needs to average around 3 AU per day (the helioshock is 90 AU from the star). That gets them from the setting's Earth-equivalent to the system's primary asteroid belt in well under a day, when I'd rather prefer such a journey take around a week.

A completely nonsensical - but easy to implement - solution is to set some arbitrary cutoff point (say, the snow line at 5.1 AU). Closer to the star than said cutoff point, speed is drastically reduced, to something like 0.3 AU/day or so. Beyond said cutoff point, you see a marked boost, allowing you to get to the helioshock within the requisite month - in the case of the 5.1 AU cutoff and the 0.3 AU/day speed within it, you need something like 6.6 AU/day beyond the cutoff line to make the one-month deadline.

A bit better solution, at least in my mind, is to have the boost drives slow down as you get closer to the star, with the easiest to implement being a linear progression - calculating speed (in AU/day) is a function of distance (in AU) - x - that looks something like f(x)=0.06x+0.15. This is more difficult to implement, but when simply traveling from one orbit to another, isn't too terrible. The problem, of course, is that you don't always want to wait until the point when your destination is (after accounting for travel time) a straight line away from the star, so actual travel times and routes are going to be rather complicated.

An idea to make that last option work is to allow vessels to travel along a planet's orbit around the star at an incredible rate, such that you'd basically travel like that at the start or end of the trip until you reach the point you want, and just go in a straight line away from/toward the star otherwise. I'm thinking maybe 10x the normal speed (I was actually considering higher, but then ships with more than 3 engines might be able to go faster than light by my equation, and I'd rather avoid that outside of hyperspace).

However, what unforeseen consequences might such a scheme have? At 10x speed, you can do a full circuit of your current orbit in a bit over 3 days (EDIT: This is looking at the orbit at 1.11 AU, the further away you get the longer a full circuit takes). Does this prevent any particular type of adventure type... or cause any others? Should I limit this to be within a planet's (or asteroid belt's) orbit around its sun - so with my aetheric stars, you'd only be able to get the speed boost at 0.25, 0.41, 0.67, 1.11, 1.84, 3, 5, 8.25, 13.6, 22.45, and 37 AU? Note this would let me go to something like 30x normal speed (full circuit in a little over a day) without breaking the light barrier.