> 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.
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.