"Anyway, having said that disclaimer, my non-lawyer understanding is that you can't terminate someone for a religious reason, even if it's not the only reason."
Then the solution to cancel culture is for those espousing disfavored views to couch them in religious terms, at least in part.
That's not going to work bc judges will usually view the law as broadly or narrowly as necessary to advance the judge's own agenda.
For example, courts will not generally protect bakers, florists, photographers, etc who do not wish to serve same-sex weddings for religious reasons. This should be an obvious 1A protection but it is not. There have been some favorably ruling in favor of individuals or businesses on these grounds but generally only when the govt or regulatory body behaves in an egregious fashion.
Lots of people have been put out of business when it is obvious that their activities should be protected by the 1A.