I'm not associated with FluffyChat, just want to point out that it appears to be a Matrix client that meets the "UX non-nerds would want to see" requirement: https://matrix.org/docs/projects/client/fluffychat

What's wrong with the design of Element? I don't think it's anymore complicated than e-mail and even my grandpa has one of those

Design isn't the biggest issue of Element imo. The speed it. It's super slow. I used another matrix app (can't remember the name now) and that also had the same issue. And the speed wasn't the app issue I think as everything else worked fine but loading messages and sending them took forever. Has this improved recently?

It sounds most likely the issue wasn't with the client, but with the server. The most popular server matrix.org is dealing with a lot of clients, but its performance has been improved, well, "recently", by improvements to the most popular homeserver Synapse.

The next step plan is to move to Dendrite, which is a new homeserver taking the lessons learned from writing Synapse, as well as being implemented in Go instead of Python. But it doesn't yet do everything. You can try a dendrite-hosted server by using dendrite.matrix.org as your homeserver. (AFAIK it has its own accounts.)