Interestingly enough, there's a recent theory putting the location of the proto-Germanic speakers in Finland.
There is no credible theory to that effect. Either you have stumbled on something that is not taken seriously, or you are misunderstanding the consensus. Namely, Proto-Germanic speakers did visit the eastern Baltic coast for trading and raiding, and so there are Germanic loanwords into Finnic languages of Proto-Germanic date, but the agreed location where Proto-Germanic formed is in Scandinavia, not Finland.
I'm not sure you have a good grasp on the meaning of the word "recent". A recent theory, by definition, must differ from the consensus.
> There is no credible theory to that effect.
https://www.biorxiv.org/content/10.1101/2024.03.13.584607v2
Granted, they don't say "Finland". They say "the northeast along the Baltic coastline".
There’s no new theory here at all, just some nice archaeogenetic evidence to support a quite traditional view. FWIW, I work in a closely related field and am constantly reading Germanic–Finnic and Baltic–Finnic contact literature, and I can assure you this is old-hat stuff.
You've quoted something that says after proto-Germanic had diversified, daughter lineages left southern Scandinavia to establish themselves elsewhere in the world.
But I pointed out a completely different idea in the paper, that before proto-Germanic diversified, about 2000 years before the time you mention, its speakers arrived in Scandinavia from "the northeast coast of the Baltic".
Moreover, you posted about a “new theory”, but the paper here only gives new evidence for an old theory.