I thought that since this is a relatively undiscussed topic, I thought this post might at least help some people in the future.

Thumpers and Dargyn pilots have set spawn points all over PoE, the best way to find them is to fly around on your archwing, rapidly between the spawn points.

The pilots make a sound when spawning (the same sound as when one of those ships in the sky make, you can't miss the sound, it's pretty loud and obvious). They also seem to spawn more frequently when you're doing max level bounty (although I'm not sure if this is true).

Then for the thumpers look at this article: https://warframe.fandom.com/wiki/Tusk_Thumper_Doma there's a picture at the bottom with circles. just fly between the two "often" red circles, and one will eventually spawn.

It seems that once you enter a certain area, a pilot / thumper has a chance to spawn. So by flying between two different areas you can spam the spawn 'proc'. Almost forcing these units (thumper / dargyn pilot to spawn). Make sure you fly far enough away. Flying between those two red circles is more than far enough (go to the opposite edges of each).

The more distance you cover the better. And I read somewhere that you need to be fairly close (~30ish meters <- extremely rough estimate ) to the ground for the thumpers to emerge.

Use shattering impact and sarpa to lower armour, and higher bounty lvl spawns doma while lower lvl bounties spawn bull.

Credits to the wiki article for the hot spots.