I also use both Tab Stash and Sidebery. Tab Stash is really more for long-lasting / bookmarked tabs / handling various contexts in my mind and work to get back later. Sidebery on the other hand helps me manage what I currently have in mind and to juggle Multi-Account Containers in neat sections.

That said, I've actually ended up using just Tab Stash because yeah, having too much on your Sidebery tree gets messy and now you have to deal with two sets of stashes, and yeah, unlike Tab Stash, Sidebery doesn't let you group them up into sections.

My only complaint with Tab Stash? I wish it retained which Multi-Account Container was used when it stashed a tab. It'd be perfect if it did that.

On a sidenote, I love that Sidebery lets me scroll through the tabs via scroll wheel. It's a thing I missed since I switched to a Mac because last time I checked, you could scroll through tabs on FF Linux but strangely it's not something you can do on Mac.