I'd suggest something like this:

"Zum Abschluss der Umfrage 2016 möchte ich zunächst Ihnen allen danken". Note the "Ihnen", which isn't there in English but required for an idiomatic translation.

"Dies scheint im Einklang mit der Hypothese, dass die LW-Gemeinde nicht in der Bevölkerung abgenommen hat, sondern eher in andere Gemeinden abgewandert ist". Note that the "not so much" qualifier is applied to the first half in English, but the second half in German ("sondern eher" → "but rather").

The first sentence is probably better started with "Zum Abschluss" than "Als wir abschliessen".

The second sentence is not wrong per se, I am having trouble coming up with a better translation. I'd probably use Communities instead of Gemeinden, as Gemeinde is more municipality or parrish, and the original sentence refers to online communities, which for me are not entailed in Gemeinde(in this context at least).

'Als' Is not used in that way. 'Da / Während / Nun, da ' would be more acceptable, 'So' maybe.

As a student -> Als student OK As I drove -> Als ich fuhr OK past tense As I start -> Da ich anfage, so ich anfage, während ich ...

I think it's more practical for Germans to adopt the English usage then for you to learn this strange exception;)

"Als" is not correct here. It should be "Da wir die Umfrage 2016 abschließen ..."

In the second example it should be "... dass die LW-Gemeinde in der Bevölkerung nicht sank sondern eher in verschiedene Gemeinden abgewandert ist." But I'm having a hard time translating "so much as" in this context to German.

"Not X so much as Y" is pretty much always be translated as "Nicht X sondern Y". "Sondern" should be used here.