For trees I think it is kind of subjective to what type of trees you are looking for. I have purchased these trees
http://www.vizpark.com/shop/real-trees/ and mixed some assets with Megascans. You will always have to do some material work on the trees though, but its fine.
I have never really created my own trees with GrowFX or Speed Tree
https://store.speedtree.com/ because there are so many pre-made, and I have never needed a specific art directed tree.
"I'm a little concerned there will be a visual disconnect between other people's assets and megascans"That may be true because the Mega scans assets are very good, and scanned :) But I think you can find some good free trees online and maybe purchase some specific trees you need.
I would also recommend getting Forest Pack
https://www.itoosoft.com/forestpack.php if you don't have it yet. Great plugin and very cheap too.
In the end I think you can mix and match all kinds of assets. I have made a few forest scenes and have never worried about it. I just adjust materials a bit. You just have to make sure you scatter enough assets around to look right!
Good luck!