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.
Yes but Bitcoin core does not have built in re-broadcast and the layout of Core is woeful. I do use it but, terrible really.
I thought an electrum transaction prune could happen if it totals up the ones it prunes and puts that in the file as a subtotal and then works with that and the handful of latest transactions. If past transactions are done and dusted, obviously they cannot change so it should be ok to represent the past as an added up subtotal, I assume if one were to kill the file for some reason and have to start over it again would repopulate the file with all the transactions by syncing and again do a prune to cut it back.
For a pruned file the sync would see the date stamp of the prune and where to start looking.
Damn.