Re: choosing congregations: I think you’re right; there may certainly be sad cases where you cannot find a good congregation for the belief system that fits you best, and a good congregation is really necessary. But again, I think this is a place where Seeker and Spirit need to have some give and take. It may be the case that in this single life, it is part of the Seeker’s journey to walk a difficult path alone. It is a worthwhile thing to learn…

“If there were no sense of community there, or an unhealthy one, it would be bad to join, no matter what their beliefs are. (You could still adopt the philosophy, though.)”

This may cause problems if a core element of “the philosophy”/the beliefs involves (or otherwise requires) place very high importance on joining and functioning within a community/congregation: the philosophy/beliefs may focus on/”live themselves through” participation in group events, and may even strongly prohibit “going it alone.”

For example — from what I know of Mormonism, a faith I believe you’ve had some contact with — a person who converted to (or grew up in) Mormonism, but who could not or (even for some strong/compelling reason) did not meet with other Mormons for worship or other purposes, would have effectively “zilch” Mormon life. (His/her “solo” version of Mormonism might somewhat resemble joining a football team, never attending practices or games, but working out alone in one’s room with a football and calling this “solo football.”)

Re the very important distinction between joining a belief-system and joining a congregation — C. S. Lewis’ SCREWTAPE LETTERS have some (in my opinion) very interesting and thought-provoking things to say about the difference and why it matters.
As you recall: early in Screwtape Letter Sixteen, we see the professional soul-warper Screwtape advising his younger colleague Wormwood about how to make sure that the soul in Wormwood’s charge does *not* stand the least chance of learning anything true or important from the church he attends. Here, Screwtape considers it of prime importance (for successful soul-warping) to encourage his victim to go “congregation shopping” as we might say, in order to find a congregation that perfectly fits, that perfectly pleases, but that (by the same token) does not in any way challenge his present self.

Screwtape advisest that, if the soul-warper can’t cure a churchgoer of looking for God, then “the next best thing is to send him all over the neighbourhood looking for the church that ‘suits’ him until he becomes a taster or connoisseur of churches. … The congregational principle
[of joining a congregation
because you like the congregation,
rather than the "parochial principle"
which Lewis favored -- joining a church,
finding a nearby congregation of that church
and sticking to it ]
makes each church into a kind of club, and finally, if all goes well, into a coterie or faction … [and] makes the man a critic where the Enemy
[the enemy of soul-warping:
God, or Spirit as you would say]
wants him to be a pupil.”

Further down in the same Letter, ofof course, Lewis *does* address the fact that “congregation shopping” (instead of just sticking with whatever you can find nearby) may become necessary even though Lewis plainly doesn’t like the necessity:
Screwtape notes to Wormwood that, of the two churches nearest Wormwood’s current victim, the first has a cleric who sees his job as protecting the congregation from all inconvenient truths, and the second has a cleric who runs his church on hate, often preaching “some kind of theocratic Fascism.”

(One could even go further: I suspect that Lewis would have heartily approved a bit of “congregation shopping” if the pulpits of nearby churches proved to house thieves, terrorists, or child-molesters.)