I hesitate to say this, but File->Archive may be exactly what you are asking for. That deletes non-investment transactions before a specified date, and it makes an adjustment to keep the balance as it is.
I recommend against it. It is not something that you can readily undo. You may think you are going to achieve a significantly smaller or faster Money file, but those who Archive for those reasons do not get what they hoped for. Those who do that
out of a sense of neatness, in having fewer transactions in the register, are more likely to be happy with the results of archiving. You give up the ability to make reports over longer periods.
Maybe balancing (reconcile) the account up to a point, and to only display unreconciled transactions in the register would be useful to you.
