My first post, but have some experience in electrical and now lately the WAC brand. Transformers need to be readily accessible, such as back of cabinets, basement, or in a closet. Locations that would be frowned upon: Above upper cabinets requiring a ladder, underneath bottom cabinets requiring kneeling and removing toe kick. Therefore, put it wherever it meets your design criteria.

Can mount transformer inside cabinet but depending if it is a plug-in or direct wire application. If plug-in, then need outlet gang box that is fed from non-accessible part of the cabinet. Direct wire would require conduit into a knock-out.

Just note, if you can pull your lighting load from a lighting circuit, not any of kitched outlets. That is code but I doubt a 1A load will really make a difference since we are dealing with LED's these days.

Joiner cables can be run thru wall but need to be "in-wall" rated or meet CSA rating. Class 2 wiring I believe also meets. Since I've dealt with WAC, they have joiner cable that is "in-wall" rated and not. Make sure to use the correct one.

To save money, use "in-wall" speaker wiring if making a long run to a gang of lights. Thermostat wiring is good also if decent gauge. Then use their lead wire of various lengths to connect the in-wall wiring to the strip light.

Splices can be made in the wall but I don't recommend it for a couple reasons. One their connections are not strong, meaning they can easily disconnect just from their own cable weight. Second would be for trouble shooting.

Use a small gang box to tie multiple leads with wire nuts or better yet Wago terminal blocks. Ideally would be to have these connections protected inside of box but not required. That would eliminate the 3/4-way costs also.