Almost identical experience in the early 1980s, with a 2x converter and a 135mm Tamron lens. The effective f-stop was 2x worse, and the quality towards the edges was iffy to say the least.This is from the days of yore, when even 400 ASA colour film was a novelty - but I bought a 2x converter and used to use it with a 135mm f 2.8 Soligor (and, I think a 180 mm f 3.5), but only for a short time. The results were crap. Cheap, but nasty, I'd say.
They don't "alter the amount of light coming into the camera" - it's just plain physics. You have a prime lens of a certain f stop, and once you put a 2 x on, you have the same amount of light actually going into the lens, but are 'zooming in' on what there is, so you have to open up two stops. (To get the same amount of light from the subject matter, you would have to add two stops to the prime lens - that's why long lenses with wide apertures are awfully big in diameter, heavy, and expensive.)So save the pennies for a bigger lens.
I’ve been doing research on the internet on them.
Apparently they alter the amount of light into the camera.
Coming from a bridge camera there is so much to learn. Hopefully not financially.