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.
Not possible. Electrum has to use your transaction history to calculate your address balance and script the transaction. If you were to prune the transactions, Electrum has to query the servers repeatedly and this is rather inefficient.
If you have such a powerful rig, you should consider a full node, like Bitcoin Core. Unlike Electrum, it doesn't have to query the servers which creates a bottleneck. By doing this, you are also helping to save the resources that would otherwise be used. Electrum is honestly not designed for heavy usage.