Any solution for footnotes outside text frames?

Have you checked out @Peter Kahrel's footnote scripts? Maybe Footnotes to Margin Notes (with multiple placement options)? Please note that these are donationware, so if one works for you, please don't forget to make a donation. 

https://creativepro.com/files/kahrel/indesign/footnotes.html

~Barb

Thanks all.

The thing about footnotes is that they follow along. Early in a book that's pretty important.

Unfortunately, I'll have to find some workarounds.

Use endnotes as you can place their frames anywhere you want,

ID's TOC, Index, footnotes and endnote features have considerable formatting range, but are more or less locked to single, inflexible structures. You can use Cross-References fairly flexibly, though, and you could build an alternate footnote schema using it.

But if this is for print, you don't need any feature. Just set it up and style it manually.

If links are needed for electronic docs, crossrefs, combined with some artful styles, especially object styles, and some scripting, are about the only approach.