A possible fix for "submarine income"

[Jojoya]

Solution: ceruleum fuel becomes untradeable.

Better solution: remove ceruleum tanks as an item. Pay for voyage costs directly with company credits at the time the sub/airship is being deployed.

I think the damage is done already to any small adjustment have meaningful impact.
Your proposed changes would achieve that people will stock themselves and buy in larger amounts to compensate any delay/timer.

It would hurt the small fish, but the big players are stockpiled for next year(s). Ultimately it would favor the biggest fishes in the pond by culling any growing competition.

Make voyage control panel accessible only with endwalker MSQ finished. Square will cash on these main scenario boosts.

Small fish wouldn't really get hurt by this change since they're not buying their fuel from others in the first place.

It may not impact sub farms that are already operating much but it would impact the addition of more workshops or creation of new sub farms.

I like my idea better since those that had been stockpiling fuel in advance would have ended up wasting all those company credits on suddenly useless tanks.

SE is extremely unlikely to force FCs to have members at level cap to run the control panel. They don't take existing content and turn it into max level only content as much as some players would like for that to happen (hunts and S ranks being a good example).

Another and easier fix is just adding the ability to run them for anyone via Grand Company or similar, it would balance the market.

That's not really the OP's concern. The OP is concerned about the impact of the sub farms on housing supply.

It also wouldn't do much to balance the market as long as Salvaged/Extravagant Salvaged Accessories exist. Giving every player access to a workshop would lead to increased inflation across all markets once they ranked up their subs enough to make accessory farming viable.

It's also really not hard to farm credits in a day if you really wanted to (spamming crafting/expert delivery/alt character progress, etc.).

I agree that it's not hard to farm credits if you're only concerned about one or two workshops but on the scale that would be needed for a single player to run 20-30 workshops? That is not so simple.

Certainly they could turn to botting for the credits but that comes at a massive risk of losing everything. The sub farms get little to no attention from SE. Bots do get attention even if it's not sufficient to satisfy the player base.