Ok, I tend to take a "old farmer" approach to problems or projects like this. Personally I would first off spread a high N fertilizer and then plant a N fixing cover crop, such as clover or alfalfa. When your thistles get up good, just before blooming, spray the entire plot with a mix of Roundup and 2-4D amine weed killer. ***Note: please be sure to wear appropriate protective gear*** I usually mix my weed killer at 2oz of Roundup and 3-4 oz 2-4D per gallon of water. Leave it sit for several days, then go back in and work it all up good. You may need to be aggressive with the thistle, Canadian thistle is extremely hard to eradicate once it is well established. I can remember on the farm fighting the same patches year after year just to keep them from expanding. Only thing that really knocked them out were some of the chemicals that are not available to the home gardener that have very long residuals , which you don't want if you are growing broadleaf plants lol. Digging thistle will never eradicate it, as within a year or two there is so much seed built up in the soil, it just keeps coming back. Anyway, I would repeat the above process for 2 years, trying to get 2 cover crops a year in, and see if that doesn't rejuvenate it a lot. Then I would go with a soil sample and adjust what nutrients you add at that point.
***Disclaimer*** I know and understand that a lot of people are leery of using the weed spray type chemicals. I personally feel that as long as you follow label instructions and use appropriate PPE, they are safe and effective. But then again, I've been using them for 30+ years, and old dogs don't learn new tricks
