There are several ways to approach this, as there are people, situations, climate etc. For me, I am trying for a living soil in my pots. I have herbs (sell fresh cut at market), flowers, a few veggies, 100+ pots from 12 -16". My climate probably similar to yours, zone 7 and 50+ inches rain, but do have hotter summers.
As I have mentioned several times, try to find a top soil and bagged poo with sand to be part of the total mix. This keeps your pot draining and loose. There are also earth worms in most every pot. Sometimes your old plant is pulled out in fall, say tomatoes, but you can leave a bunch of root in there as organic to break down. Some plants left in for the winter, such as parsley, then pulled in spring, top few inches of soil refreshed, plant new. I dig down enough to check for worms and look at soil. For perennial herbs and flowers, I try to get around to all of them, loosening the top few inches, cleaning etc, work in a few inches of fresh and get soil lever back up. I figure if worms are active, soil loose, all is well, try to leave the bulk of soil untouched to build the living soil. Some plants you want to rotate. For example put tomatoes in a different pot. Sometimes it helps to put a plastic tag in come fall, with Tomatoes (variety) 2015, Beans, variety 2015 etc.
If some soil cleaned out from top or pot dumped for whatever reason (not disease) it gets put in my enriched dirt pile aka compost. I did dump a large pot the other day that had been neglected for several years. I dumped it in the pile and a couple of worms as big as snakes wiggled out. Yikes!
Slugs? Yuck! Next season try the Sluggo or whatever brand that is granular with iron. It's in the organic -OK to use category, but even then I try to use sparingly. I try not to have pots directly on ground or concrete. They are up on something, no saucers. I use pallets for many of them. Many smaller 12" pots on plastic shelving. Mine is from Home Depot, the shelving with grid, not solid, so it drains.
Hope this helps!