I find it really fascinating!

Yes, archaic in the sense that it can feel like a window into the past. (not a bad thing) .
One example is the word 'nog' that Finns use for "certainly" but Swedes nowadays use for "probably". That is a shift in the value of the word that has taken place over the years. A small word, easy to overlook but big difference! (Being taken for vague when infact trying to emphazise can be bad. "Yes honey, I will probably take out the trash tonight!" won't end well...)

The truly fascinating thing for me is that if you pretend talking to a really old relative, then suddenly your mind starts to decode the words in the old way and it makes sense again. This is the closest I will ever come to time travel! A true gift!

But yeah, some of the diverging is less than thrilling.
The mix of Finnish words is one thing, the other is that some Swedish words are dropped entirely and new ones are invented from raw translation of the Finnish expression. Apparently a 'pedestrian crossing' has become "skyddsväg", which looks like a swedish word but makes absolutely no sense to me. It doesn't describe anything known to me, and the vast majority of swedes would think it is a road for veichles in a military protected area, or possibly the way to get to the bomb shelter. Only if you know the Finnish word and can follow the "logic" of translating 'suoja' and 'tie' separately you can see how we ended up with this one.