I don't think the bush example on the left can be significantly improved (other than the flowers) and keep its qualities, but I think the one on the right could easily be improved a ton - the basic leaf shape there could be managed with half the anchorpoints or less, and still have significantly the same feel - especially if there won't be a deep-zoom-in close look at the leaf-element level: one assumes from your symbols & brushes attempt that the intent is to have a lot of these - like a siteplan or a decorative motif.
I'd grab all the leaves on a copy of the right one, and run the Object>Path>Simplify tool and see how many anchorpoints you can lose whilst keeping your same basic feel: I'm guessing you can cut your anchorpoints by as much as 75% whilst keeping that look. I'd do the same to the flowers on the left-hand bush, BTW.
The tool gives you a live preview of the impact of your impending edit, and a rough estimate of the impact on the anchorpoint-count on the given item. I'd do one leaf first as a tester, and provided you're OK with the results, move on.
As you can see, less than half the anchorpoints and still enough of the same shape and feel to pass muster - and my initial anchorpoint density was about a tenth of your right bush.
If the viewer will never see the leaves close-up, they should really be held by only 4 anchorpoints - that's all they should need for a basic elongated / tapered ellipse.
Only after you've done this type of cleanup to all your art would I make my symbols of these - and yes, I would do that for sure: symbols will really help keep your filesize down.
Hope this helps.