For production quantities, I use JLCPcb (China) with Airborne for shipping (so it doesn't take a month to get here). Their quality is very good. Pay the extra $20 and get a stencil to make paste application easy. It's worth it. I think my last order of 50 board roughly 2.5x3.5" with a metal stencil came to about $70 total with an additional $30 for 3-5 day shipping via Airborne.

That's why I made a lot of boards myself as described in my comment - I get film exposed at professional house that does it for offset printing, and this is pretty fast and cheap (about $3-$4). I pick it up myself. The rest is done with photosensitive 3M dry film and Chinese wet soldermask. Drilling on DIY CNC.

Sometimes I get 4 revisions in the pipeline where 3 versions I haven't even received yet are already obsolete and go right from the mailbox to the trash.

Even the 4 layer board processes are not set up for good power integrity. They have a center core for layers 2 and 3, with 1 and 4 as foils. It ought to be two cores with prepreg between, giving the low impedance for internal power planes.

Generic fiberglass is NOT consistent across multiple mfgrs enough that you can spec it for mass production from any mfgr in any country at any time, you need the very low variation specified "RF PCB material" to mass produce precision untuned hairpin microwave filters.

HOWEVER, the cool part of the project was generic fiberglass is consistent enough across one mfgr over a short period of time, that you can characterize product X shipped from PCB house Y and then you can very consistently get boards at the proper resonant frequencies.

The meme was not overly successful in the amateur radio microwave community, sadly, but the project did work perfectly well at the time.