You don’t fly an industrial command ship for it’s personal yield, you fly one to boost the yield of other ships in the fleet flown by your friends and/or your own alts.
Some numbers I got from quick simulations in Pyfa:
(I assumed maximum skills, T2 equipment and mindlink. The numbers will be different than you have.)
Max ore yield from Porpoise with industrial core, 5 mining drone IIs and drone rigs: 14.9m3/s
Max ore yield from Retriever with T2 strip miners type B crystals, unboosted: 30.7m3/s
Max ore yield from Hulk, same setup: 50.2m3/sMax ore yield same Retriever, boosted by Porpoise: 51.7m3/s (of which +21.0m3/s because of Porpoise)
Max ore yield same Hulk, boosted by Porpoise: 84.6m3/s (of which +34.4m3/s because of Porpoise)
As you can see, the yield of the Porpoise is low, and is less than the boost it gives to a single other barge. The yield of the Porpoise itself is even less than half the bonus the Porpoise gives to a single Hulk next to it.
For the mining yield of the Porpoise I wouldn’t use one. But, from the numbers you can see that a Porpoise + boosted Retriever mines more than two unboosted Retrievers. And a Porpoise + Hulk mines about as much as two unboosted Hulks.
So, for two characters it already breaks even to put one of the two in a mining command ship. If you have three or more ships in your fleet, the fleet as a whole massively benefits from having one of the characters in a mining command ship, from the added yield alone.
Don’t fly the Porpoise to mine ore, use it to boost your mining fleet. I probably wouldn’t even bother using drone mining rigs on it.
As to how you ‘tax’ the mining fleet, I guess that’s up to you. Some players with multiple characters won’t mind putting one in a mining command ship for the convenience and extra ore it brings. If no one is willing to do so, perhaps talk to your friends what a fair tax is.