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You can try to stop the Trezor bridge to force electrum to connect to Trezor directly via USB (or vice versa). Maybe that helps.
A maximum of 5 inputs sound very little. In the past, I had no problems with 50 inputs, at least under Linux. With 100 inputs it starts to get slower (especially if rvn doesn’t use segwit). The problem gets worse if the mining pool also used large transactions, because the input transactions are sent to the Trezor. I’m not aware of a timeout, is this something electrum added?
Edit: Thinking about it, the timeout could also be caused if some packet gets corrupted and the library is waiting for a reply that never comes. In that case increasing the timeout wouldn’t help. Maybe a better cable helps, or maybe it is another application that steals your Trezor.