I’m aware that this will be an unpopular opinion but I’m honestly unconvinced that Congregations should have any assumed, default and direct role in missionary efforts to other nations at all. Any role they have should be at a remove - for example, if sending Liao to foreign enclaves of the Way were a tool for helping them, meaning that those with Congregations would have best access to such Liao in the first place but opening it up to others as well.
Congregations are already a pretty meaningful personal resource because they slide you into one of the houses of government just by possessing one, so I’m also unconvinced by arguments that Congregations are dull and need to be improved by having more options like that.
A number of tools already exist for uptime support of notions of missionary efforts and religious spread; I would suggest that making them coherent and explicit would be far more valuable than trying to work entire new downtime options in. For example, we have the magical capability to send letters and materials via magic if we know the name of the recipient. Therefore, I’d like to see messages from named missionary leaders in foreign lands coming in, either to the Synod as a whole or to specific Assemblies and Cardinals as relevant, saying ‘hey, we’re doing X, it’s going Y, we could really do with Z’ where Z is often going to be ‘please send us liao so we can roll around in the purple stuff and binge on holy drugs while showing the locals how awesome it is’. This would allow general citizens and priests to, as individuals or groups, get together and gather funds for overseas missionary efforts, sending them in uptime via ritual. The key things needed to make this happen that we currently lack area) names for recipients b) a regular feedback element via messages or whatever.
It’s also clear that missionary elements can trivially be represented through the existing system of works that the Senate currently handles; the issue here is that the Senate handles all of it. While on the one hand I could see the Senate handling a big motion to get political permission to build a religious enclave in another nation, giving a target that players can come together to pool their resources to achieve, It would be nice to have some of these kinds of things come straight through the Synod instead; a surprise request to ship 10 wains of White Granite to our missionaries in Weirdland where things are going very well and they need to build more shrines (or possibly they’re lying and actually they need the stone to build ramparts because hostile natives are attacking, and this will cause a political shitstorm down the line when the rulers of Weirdland start sending emissaries to the Empire asking why the fuck we’re funding insurgents building fortifications in their territory).
I would also note that a key place to bring elements of missionary and religious work would be newly conquered or reconquered territory, and a sort of ‘spiritual/pastoral clean-up’ process of bringing in resources and manpower to such areas would be one of the better ways of actually representing the spread of religion by fire and sword.

