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The west coast of Ireland will suit your requirements as long as you select a town or village that has what you need.
There can be a heat wave but even then the temperatures are relatively low, 20's mostly.
The father north, the fewer tourist and a bit inland you will find even less tourist while the weather is still cool.
As Ireland and Germany are both EU, you should not have problems with working there. You may have to register.
The west coast of Jutland, Denmark will likely also be within your temperature limits as are parts of the west coast of Norway and both those are EU or EEA and not to hard to arrange long term relocation and remote work permits.