Swapped battery now clutch won't disengage.

[Ognog]

The condition of that fluid tells you something....yuck.
I would plan on taking the swing arm off, clean and lube all the needle bearings in your linkage.
Pull the forks off, remove the steering stem and lube those upper and lower bearings.
While the forks are off, drain and refill with fresh fork oil.
Inspect the wheel bearings, remember you have 3 on the back. (2 wheel, 1 cush hub)

Your bike will thank you.

I would love to extensively go over the bike so I can have that reassurance and confidence as I'm riding around town I just really don't have the means (tools and a garage) or knowledge to go about doing that.

You could send it to me and I can do it for you. I have a workshop, tools, experience and time. OTOH it might be cheaper to take it to a local shop.

All the way in Germany haha? Cost more than the bike.