You have to just wait for new listings sometimes, but an underrated brand favorite of mine is Sector watches from the 90’s. New old stock of good ones pop up from time to time, especially coming from Italy so it’s worth searching for them in French too lol. The watch I wear the most lately is my Sector 450 diver watch I sniped someone in an Ebay auction for and got for $32 with tax and shipping. This is the same watch listed for $130 and honestly even at that price it’s a good deal. The sapphire crystal is very thick and the watch itself is only 8mm thick, 7mm excluding the caseback. The bracelet is top notch and like most 90’s Sectors has a diver’s extension clasp with microadjustments. You never see it but the movement is actually pretty for a quartz too, with vibrant blue circuitry and stainless steel construction.
A brief(ish) history on Sector and why these tend to go under the radar and sell for below their worth. It was founded in 1973 by Filippo Giardiello in Italy, and they were most popular in the 80’s and 90’s filling the niche of sporty and masculine diver watches. They were primarily manufactured in Switzerland with Swiss movements and branded themselves as Swiss watches, around Y2K they went under and in 2001 sold to BVLGARI. Sector operated under BVLGARI until 2006 when it was sold to The Morellato Group, an Italian jewelry company that had no experience with the watch industry that was looking to expand their business and try to pull a Cartier… in the early days they did release some decent albiet lower quality watches, but they absolutely tanked the brand into obscurity with boring, unoriginal, uninspired watches from questionable origin and they did it right as smart phones were coming out and social media like Reddit was beginning to exist
Tissot still makes pocket watches that almost fit your requirements, some models for example being skeletonized with display caseback and 100m water resistance but its not under 400 dollars