As with mostly everything with boats used for essentially leisure, the more you use it the less problems there are. This is certainly true for fouling, I aim if at all possible to take Tigerfish out at least once a week even if I just go for a blast round the Solent in rough weather, great fun if you have that kind of mindset...
For stopping fuel consumption rising through the year aside from regular use I find the most effective thing is cleaning the props, I do this with a brush and abrasive pad while kneeling from the swim platform, generally twice a year, fairly quick if they are not too bad to start with. For the rest of the leg I've found a single good thick coat of Trilux is just enough for the way I use the boat but then again I'm not berthed in Cobbs so can't compare.
Interesting that you use Interspeed Charlie, I didn't realise that is ok for aluminium.