Ok so looks like people are still taking advantage and getting multiple houses.

[Jojoya]

This is pay to win. Ruins economy and others enjoyment.
When you have a method to print Gil with workshops and gardening, you can run a monopoly.
It disincentivizes other to try. If i am a single person, or a single FC trying to make some gil. I will have to compete with someone who owns multiple of anything i could ever think to produce.

I don't think you understand what a monopoly is. No one has a monopoly over anything in the game. It's far too easy to produce most items on demand and mess with those trying to control a commodity. Some have more than their fair share but that is not a monopoly.

If you don't want to try, that's on you. There are plenty of other ways to make gil. Workshops might be a consistent way to make gil for little effort once the front-loaded effort is out of the way but when I'm putting a little effort into my crafting I'll make more on a daily basis than someone running subs for Salvaged Accessories.

What ruins economies is the game not having sufficient gil sinks to draw the excess back out, not how much any single player has access to.

That aside, SE needs to fix the system. Enough is enough. Make certain that every player that wants a house can get one, and that some players aren't hiding behind FCs to get multiple houses so others are blocked from enjoying the content fully.

I know a lot of people who got their subs up and running and quit the marketboard entirely. 0-400k every other day was a lot better than the frustration of dealing with marketboard bots.

I'll never understand people. The way to make money is to sell things that people want but others aren't selling, not go head to head with others in a saturated market (whether bots are involved or not). There's consistent demand out there for lower level items that have caused sale prices to increase because there aren't enough sellers meeting the demand. Some of that demand is so high you can buy the items straight off the NPC vendors to resell for profit.

On the submarine issue, I wonder if the easiest change is just making fuel non-tradable. Genuine FCs won't be affected and can retain the needed benefit of being able to fund their operations, but the more solo FCs someone has doing it, the greater the cost and larger the effort.

Put in a 30 day requirement to buy the non-tradable tanks. Or better, make all tanks in inventory dissolve if someone leaves an FC with any.

A housing primal would need to grind many millions of GC seals a day to buy all that fuel, rather than just buying up people's spare fuel cheaply and transferring it around.

The easiest way to solve the problem (assuming no technical barrier) would probably be remove the fuel and set the voyage cost as company credits directly. It wouldn't make it impossible for the workshop outfits to run voyages but it would cut into their profits a fair amount due to the need to get items to turn in for Expert Delivery (unless they're also running crafting bots to make that easier). Trying to do that through normal effort (retainer ventures or otherwise) would be prohibitive if someone is running 20+ workshops.