Yes, the beds are permanent but I think digging is probably required, at least for a while. The soil in a very large area is solid pan about eight inches down due to compacting by heavy machinery
I see people using lasagne method on virgin ground, just piling organic matter on top, but I too favour an initial dig, particularly if there is a plough-pan or similar, or horror-of-horrors a new-build with all the builders rubble and muck just buried under some topsoil.
But I personally would not plan to dig it more than once. I created lazy-raised-beds on mine, just digging the paths out a spit's depth and putting the soil onto the beds - so not so much raised-beds, as lowered-paths.
I think if yours is sitting under water for prolonged periods that would contribute to compaction, and lowered-paths might solve that? Increasing the organic matter will help reduce compaction too of course, and it you have access to plenty, even if just a one-time flying-start, that would be my choice.
On the issue of weeds, we're surrounded by abandoned farmland. Weeds are a fact of life all year round, unfortunately. Bindweed, wild rocket and fat hen are a particular nuisance.
I have the same problem, farmland all around me. I have experimented this year with about 50% of the crops (the ones planted later in the season
) grown through woven weed suppressing membrane, and they have been very easy to look after - no weeding, and reduced watering - so next year I will grow all crops through woven membrane (except perhaps things like Parsnips that would be very hard to harvest).
If I were to put down muck as you suggest, how thick a layer would you recommend and when?
4" - 6", more if you like, particularly if you will rotavate it in. I don't think there can be "too much" in Year 1 - you might want to put some Nitrogen down, to help with decomposition, otherwise that is perhaps going to rob some Nitrogen from the soil.
Would you do the same for the legume beds?
I would - it will help with water retention. You could dig a trench and fill that with organic matter instead, but if Beans are part of your crop rotation I think that improving the organic content generally, rather than specifically in a trench, would do much the same job, and benefit crops in subsequent years more.
If you are going to mono-crop beans in a permanent bed then perhaps a Trench of organic matter would be better.