I'd suggest just lowering the accessibility bonus instead(to 5%, maybe?), but that would mean that the first one built would be rather useless, basically making it a bonus that's only worthwhile to those who are already wealthy enough to freely build multiple stations.
If possible, I think the best solution would be implementing diminishing returns. E.g. 10% from the first one, 7% from the second, 5% from the third, 3% from the fourth, and then 1% from the fifth onwards.
If we started to dig that deep... Why not differentiate bonuses entirely (instead of decreasing bonus increase rate), based on amount of hubs available in a system? I try to explain, but you should consider this is a
bad example, I just unable to propose better:
1. First hub in a system will provide nothing than stockpile size increase, similar to waystation do.
2. If you have two hubs in a system, both will achieve bonus 1 and additional in-factional import capacity. (not sure if it actually possible to adjust).
3. If there are 3 hubs in a system, all stations with them get bonuses 1 and 2 and +20% extra accessibility for stations with hubs. All non-hostile markets in systems gain flat +10 accessibility bonus.
4. Network of 4 hubs will provide all the bonuses 1-2-3 and additionally increase burn level of all non-hostile non-military (in other words - traders only) with a factor of 2. In the system, of course. Alternatively - increase fleet strength for non-hostile trade fleets, generated in that system.
Well... Yeah... That is terrible...
But the point is: give entirely different bonuses, based on amount of hubs, and not get hung up on accessibility only.