I spent quality covid time last year ridding my acreage of about 900 buckthorn trees plus other assorted invasives. Trees ranged from 1/2" to 6". I tried everything, but only one thing worked well, and a year later, no combacks (buckthorn spreads by roots and suckers and just branches more when you cut it.) See picture in previous post.
Tools:
-- 20% solution Roundup. Get the 2 gal jug of 41% glyphosate from Tractor Supply and dilute in half. Use backpack or handheld sprayer. Tryclopr (Garlon) and Tordon also work, but considerably more expensive.
--Good lopper, like Fiskers with Titanium blades. (But they are not as good as they used to be.)
--Cordless reciprocating saw with Diablo pruning blades.
Depending on trunk thickness, lop or saw the tree at ground level or below. Good loppers can cut 2". Spray stump with Roundup. Move to next. Repeat. Helps to have two folks, one cutting, one spraying. If most of your stuff can be mowed, then mow it, but keep in mind that you may create punji sticks out of the saplings. You may need to go back and lop them and spray.
Continue to mow to prevent re-starts.