To be honest, anyone would be depressed about their plots if they were still fighting the weeds to the extent you say you are after that many years.
Can you not look at different approaches for your plots? You mention that one plot is only fruit trees, so do you have a weed problem on that plot? If so, you should be mulching to suppress weeds, using a good quality weed suppressing membrane and mulching with grass clippings or bark chips. Cheap membrane will allow weeds to get through. I would also recommend you use cardboard under membrane in areas such as around the fruit trees which will give you added protection against weeds.
Adopting a lower maintenance approach will also benefit you. I lost all the grass on my plot other than the surrounding shared paths which we have to leave as grass as we don't have time to keep on top of it. The suppressant is topped with bark chips which needs replacing approximately once a year and prevent the weeds further.
We don't leave beds empty to stop weeds growing, and use intense planting with less gaps in between for the same reason. It means feeding the soil well and manuring or adding soil conditioners whenever we harvest prior to replanting, it also means planning and keeping on top of the succession planting, so there are always things in modules as well as stuff we direct sow.
Cover areas that you haven't cultivated with black tarpaulin or similar to keep down weeds, use the lasagne bed approach for new growing areas or to boost existing ones, lay down a thick layer of wet newspaper and mulch with grass clippings then plant through to keep weeds down.
It is annoying when you have other plots seeding weeds over yours but it is the nature of allotments so it is something we all have to deal with.
Ants - I have a small camping stove which I use to brew my cuppa and whenever I brew a kettle, if needed, I use the leftover boiling water and pour it on an ants nest or large colony of wood lice that might be causing issues.
What weeds are your main bugbear? We get couch grass, bindweed and thistles on our plot and they all have brittle roots that tend to propagate easily if even the tiniest piece is left behind. Cutting out the light does weaken them enormously and they cease to be a real problem. I don't really use weed killer for large areas, only on small patches of pernicious stuff that I have tried to remove by other means or which has grown through the membrane (couch in particular with its spears of new shoots manages to get through easily so is easily killed with a quick spray with glyphosate once a year).