Show summary of artists on your pixiv bookmark page. It shows summary of the current page, and adds to it as you navigate to the next page.
That, or you can click Fetch All to fetch all of them at once. Mind that it only fetch for the current public/private setting.
If you have fetched all of them. You can choose to follow all artists which you have bookmarked a lot. Or, unfollow those with few bookmarks.
Some functions based on or shamelessly taken from https://github.com/Ziqing19/LabelPixivBookmarks