Forcing all characters on the same service account to be removed from a FC just because the player wants to transfer one of those characters to a different world/FC would be an extremely bad move.
Attaching ownership of the FC house to the FC master would work much better than the current "purchased and maintained by character" provision. Those running the workshop farms would either have to have a separate service account for each FC house or have those houses on different worlds. Someone could still hold an officer rank in multiple FCs on the same world, they just wouldn't be able to be FC master of more than one.
I'm not certain your final suggestion would be effective in any way. Even without a boost, someone could easily get a new character to 2nd Sgt. in its Grand Company within a couple of days then spam Expert Delivery turn ins to meet the remainder of the credit requirement. SE would also have to add a tracker to the game that displayed credits earned by each member. That in itself could lead to toxic behavior in some FCs ("I'm more deserving because I've generated more credits than you").
It would make more sense to move the Land Purchase entitlement from Rank 6 to Rank 20. Back when housing was first added, and even in HW to some degree, reaching Rank 6 was a difficult task unless it was a large FC because the amount of company credits generated was smaller. As expansions have come and gone, the amount of company credits being generated has increased multiple times compared to the original especially when it comes to Expert Deliveries.
A small but active FC with 4-6 members could still get the needed rank in 1-2 months (they're already going to have to wait 30 days before they can buy so it works out well). A solo FC would have a much harder time without spending a crazy amount of gil on the mb.
Another option would be to charge FCs rent in the form of Company Credits. Genuinely active FCs (once they've been established for a few months) are usually swimming in credits even after what's needed to run the subs and airship, and buying FC actions. Credits build up much faster than they get used because there's not anything useful to spend them on outside of the actions, ceruleum tanks and maybe getting Voidrake/Althyk Lavender seeds to grow.
Solo FCs, once again, are going to have a harder time generating the ongoing supply of credits without spending large amounts of gil. The more expenses there are to offset the gil generated by a workshop, the less attractive it becomes to run workshop farms.