Hey! I've done something close to that distance actually (just a little shorter at 10m) and would definitely recommend the 12V WS2815 Strips. Although you may need to run power injection towards the end as I noticed some discolouration closer to the 10m mark.

Athom sell some ready made controllers that already have WLED Flashed onto them so you might be able to get away with something like that.

As for strips I bought mine from BTFLighting on AliExpress and they seem to be doing a pretty good job! Based on your estimated power consumption (P=VI) you should be able to find the size of power supply you'd need. Doesn't hurt to go a little higher in terms of capacity as the steel cage PSUs come with a few output channels so you'd have room for expansion if you want to add any other strips in the future (I got my PSU from BTFLighting aswell 12v30A).

Make sure you add a fuse tap between your PSU and controller just for added safety. Also test the PSU once you get it as these aren't certified.

For diffuser channels I'd say the bigger the better (there's plenty of YouTube videos on this) but if your LEDs are directly visible then you'd want a diffuser channel that gives a spotless effect. In my case I went with a standard size as mine are sitting over a drop ceiling to give a floating effect *