As I see it due to the loss of being able to fit capitals in a astra you are losing functionality, I would suggest adding a service module to allow you to have limited space to put capitals in so they you could fit 5 - 10 capitals this could also be used for the Fort to fit a limited number of super caps.
Because with the POS you could use it to log off your super caps and with a capital SMA you could fit a few capitals, for the Astra to be a good replacement for POS’s you should at least be able to have limited capacity for capitals.
The bonus for the Fort for storing capitals would be two fold this way first you do not need to expect fuel to store them and second you get infinite space. The same bonus would apply for super caps to the keepstar.
The model could be expensive to run and buy furthermore I would make it as just another tab which you can set right to access it, furthermore if there is not space in the capital service tab you cannot leave your capital, so in that way it is almost completely a replacement for POS’s because you can log off in the Astra but if you do not have access or there is no room you will not be able to leave it behind or change ships.