And another follow-up. I just spoke to a local retailer asking about the
lever connectors. They only sell them in bags of fifty, but in any case he
advised against them, especially for a connection that's going to be sealed
inside a wall. He recommended a conventional round screw-terminal junction
box, either the usual size or a 2.5" diameter model.

He has advised you incorrectly, you are not allowed to use screw connections in inaccessible places ie a stud wall. Wago type connectors are approved for inaccessible places within certain current limitations which a lighting will not exceed. The Toolsatan versions are available in packs of 10, however read the reviews most of the "sparkies" agree they are not genuine Wagos and point out the spring tension is much weaker and as a result do not recommend them so buyer beware!

Richard