For thistle farming I try to find a large stand of Black Forest biome, I mark each thistle with a dot (no label just a dot). Make sure you are zoomed in as far as you can go to accurately mark the spots.

Onice I have marked a whole forest I simply go back after the thistle has re-spawned (which takes 4 hours according to Valheim Wiki).

For rasberries and blueberries I just hunt for them as needed.