Thinking further is there a function or can we add a function to trim the saved history to a user nominated value, say "keep last 20 transactions" and then the file gets trimmed.
Not sure if such a thing is possible.
You
have to create a new wallet. That is a completely new set of addresses and a different seed. Electrum just isn't designed for massive wallets. After you have properly created the new wallet, including noting down its seed, you should send your money from the old wallet to the new one.
You can create a new wallet using file > new/restore. You can open specific wallets using file > open or the command line switch -w
electrum -w /path/to/walletfile