I'm with kurplop about the tray - they can make a lot of difference if you have any shoulder issues and, well you've already hit on the negative slope.
Every 4k I've tried that's more than a year old had incredibly stiff keys - a new one will feel a lot lighter - you could replace it and then replace it again in a years time...
It is possible that you are just a bit more tense while adapting to the layout of the teck (I could be wrong). If you fancy giving it another chance, you might want to introduce a few more rest breaks in your day.
If none of that works out, other mechanical keyboards exist... If you have the cash, we can help you spend it.