I've thought this over more, and there are a couple of other things which will come into play:
1. Tankers will take a long time to refuel. Even a small tanker with 100 HS of fuel will take 100 hours at low tech. As time goes on, I expect that the growth in refueling rate will outpace the growth of the typical tanker, but it's going to be a problem. Allowing tanker systems to refuel the tanker itself from the planet will go a long way to solving this (particularly if tankers can have multiple fuel systems).
2. The requirements are going to be annoying for frontier planets, because of the size and cost of spaceports. I can think of three solutions:
A. Add in a new structure, of maybe factory size, which serves as a fuel/ordnance hose for maybe a single ship at a time at the normal rate, or even a reduced rate, and multiple structures only allow more ships to rearm/refuel. That way, you can set up forward fuel/weapons bases, and refuel ships at your new colonies without hauling out huge spaceports.
B. Allow refueling systems to transfer fuel to/from the planet. In this case, you'd just have to set up a station ship/tender for the new colony/forward base. It also solves Problem 1.
C. Allow very slow transfer of fuel without refueling systems while stationary, both between ships and between ship and planet. Maybe 5-10% of normal rate, or even a fixed rate of say 5,000 LPH. This is slow enough that it's not likely to be abused, but also means that when you make a mistake and need to rescue that ship, you don't have to divert a dedicated tanker.
All three do slightly different things, and could be implemented together.
3. The refueling systems could be made more interesting. What about two different small versions, one that is a military system and is intended for fighters (10% size, 10% rate, 10% cost), and another that is civilian, and doesn't work underway, for point-to-point tankers? The latter combines with 2B to serve as a secondary fuel handling system for the unrep ships. It could even transfer at different rates to/from planets and ships.
Actually, that's another question. Do refueling systems allow transfers both ways? Could you use one to pick up fuel from a ship and move it to another ship?