Congratulations! You've reinvented the wheel called RSS.

Not sure why browser vendors like Google prefer the idea of a separate addon for each site which periodically crawls and parses html (yuk!) and kill support of RSS & Atom in favor that weird idea.

Fortunately e621 still supports it even with tag exclusions.

Don't worry about the limit of excluding only 9 tags per subscription, since you could have an unlimited amount of RSS subscriptions, you could scale it to one desired tag with 9 undesired tags per subscription.