I had another idea of just using a old Laptop with a fresh Linux Mint system to manage the wallets and transactions and nothing more. Risks of malware/trojans would be close to 0, not only because of Linux, but because of no other usage.
If you have a laptop for this job, a laptop that you can guarantee it will never ever go online, you can make yourself a nice cold storage.
All you'll need is a mean of transferring the unsigned transaction to this and the signed transaction from this. Usually an USB stick does the job, although some argue that it's not 100% safe (imho it's close to that though).
Although it's done for a previous version of Electrum, this doc will help you to understand what you have to do:
https://electrum.readthedocs.io/en/latest/coldstorage.htmlRemember, the laptop that holds the seed/priv.keys has to never ever go online.