I don’t get the impression that the OP was intending to only make a trade every 3 months. I think that timeframe was chosen because of the 3 months time you have to wait between cancelling and re-joining plus, so he was saying he’d buy it for a month, cancel and then re-join 3 months later.
It still seems like an awful lot of work for someone who said it’s too much of a hassle to just move £4000 of his savings to FT to get the plus features for free, but each to their own.