Interesting that you couldn't find spark plug sockets that fit. Metric ones are rare, but the equivalent SAE ones work fine. For 21mm, use a 13/16" socket. For 22mm, use 7/8". The latter can be a little hard to find, but an oxygen sensor socket works too and 7/8 is more common there.
https://www.homedepot.com/p/Husky-3-...1316/202913555
https://www.autozone.com/test-scan-a...cket/47738_0_0
The Schwaben/ECS strut nut socket is still better so that you can torque it down properly.
The DIY you linked is for a RWD model vs your AWD, which might explain the discrepancy. That said, Koni supplied 19mm instead of the 22mm that I read everywhere (might have been the difference between special active vs yellows), so I faced a similar situation. I ended up using an OE 21mm strut nut that's originally for an E60 xi model. Happened to have one to check the fit and then got two new ones. Already had the 21mm strut nut socket from the e60.