There’s a few ways of making Swiss bank money outside of Broker requests.
One option is to have the Building Permit edict active, but this has political downsides and increases your building costs. Also, if you’re in the late game and not building much, you won’t make much money this way.
My preferred method is to wait until you’ve got a well-developed island with a strong economy - perhaps in a campaign mission once you’ve completed all the objectives - and use Banks. If you research the Bank you unlock alternate work modes, and Slush Fund will divert money into your Swiss account.
With two Banks set on Slush Fund I managed to get this relatively quickly. Just make sure that you go the whole mission without completing any Broker requests, as that will void the achievement.