If you intend any sort of replanting, you are going to need to work on the soil anyway. Water very well the day before you plan any work and that should help to loosen things up. Dig up anything worth saving or replanting and store in containers (I use plastic grocery bags) for the interim. Then work on improving and lightening the soil by adding in organic matter like compost.

It really makes no sense to use a herbicide to kill off what should be desirable (if untended) plants. Save those chemical big guns for the next to impossible to controls invasives or noxious weeds.

To start over from scratch you really have to start over from scratch. That means digging most everything up to make a blank slate. Sure you can poison everything dead, but planting new perennials on top of half dead roots and crowns of the old isn't going to be easy by any means.

So yes water well to make the digging easier and save for replanting what is worth saving. Many herbaceous perennials will actually respond very well from division and replanting of fresh starts minus all the old crown.

I hear you though. I have two out of control perennial beds under my care that need redoing and the thought of it is daunting. All the self sowers the Lady of the House added only makes it more daunting because that seed bank will have to be dealt with. You can hire some skilled or supervised muscle for the digging to make things easier.

Ditto to the advice above. I feel your pain lol. I neglected my garden for about three years due to life getting in the way, and now I am trying to rehabilitate and revamp the beds. Don't feel you need to do it all at once. Do a bed at a time, or a section at a time. That's what I'm doing and I'm not quite so overwhelmed.

That said, I would actually start with a plan, vague or sketchy as it may be. Think about how you want it to look, what kinds of plants you want (for instance, I'm transitioning over to more shrubs for less maintenance). At this point in the season I might even just clear out the weeds and replant the good stuff, put down some compost and shredded leaves, and wait for spring to do the bulk of the digging and planting. It's kind of amazing what a few months with some organic matter will do; on top of your existing-if-neglected garden bed, organic matter should make spring digging much easier.

Then I would spend the winter doing some research to see what you really want. Maybe plant a few bulbs this fall for spring color, inspiration and motivation. Of course, I just realized this depends on where you live, if you have a winter, etc. Not sure where you are but if you are in a colder zone, I think this would be the way to go.

Good luck!

:)
Dee