I plan to eventually move my coins to a Trezor wallet. But is there any way I can disable the old default wallet in my old PC, before I get rid of the old PC entirely?
Make sure that you permanently delete all files on your old computer, but if you want to be safer I suggest doing additional full format of your old hard drive, just make sure you have backup words in physical form.
Only way to move those coins from Electrum to Trezor wallet later is to make a transaction to new address generated in Trezor, but don't do it right now when transaction fees are very high.
I guess I can just set the lowest fees, even if it takes something like 22 hours for a single transaction.
With lowest fees there is good chance it will never get confirmed now in this mempool conditions.
There is no need to rush anything if you have old backup safe, just wait for mempool to cool down, unless you want to pay miners premium fees.