The area near a garden should be a fertility area that is mowed each time it reaches the flowering stage and used to mulch/feed the soil in the growing area. This takes full use of the suns energy and prevents weeds spread by seeds from spreading and weakens ones wit persistent roots.
If spray has not eliminated eliminated the thistles then perhaps it is creating conditions favorable for it instead of favorable for what you want there. By timing the mowing to the reproductive cycle I have eliminated thistle from my fields. Now I am able to time my mowing for clover reproduction.