Indeed, stranded wire would even aggravate the problem - here any building wire is solid up to 6mm2. Usually only DIY hack jobs use stranded wire for permanent wiring (extension cord stapled to the wall surface).
The best bet if you have more than two wires to connect is to get the splices out of the back box into a splice box. 67mm diameter 50mm deep boxes with Schuko receptacles can't take much more - if you're really lucky you can fit 3 Wago connectors back there but I'd never try it myself.
I've seen one guy who hacked a 100mm deep hole into an exterior brick wall and put in a splice box behind the back box of the sockets... might even have worked if he had bothered to take out the entire back of the box and not only a small section too small to get the choc blocks out of the splice box... I ended up putting in a splice box above the socket. That was a bad one... feed to the sockets (twin socket), feed to another socket, feed to two wall lights and the travelers of a 2-way circuit...
