Need advice on PCB manufacturers

Is it possible to use JLCPCB for DIY Teensy 4.0 and 4.1 projects using the bootloader chip? I would like to make a custom Teensy board that includes the RJ45 Ethernet connector already assembled. I need several hundred boards and I really prefer not to have to do any hand soldering. Would I order the bootloader chips from the pjrc website then ship them to JLCPCB or can they procure them directly from your site or somewhere else?
I've never used OshPark, I've done quotes several times as it would be nice to use a US PCB manufacturer but it never works out, their prices are never reasonable vs qty.

I've used PCBway a couple of times, I can't really remember why. I hate the appearance of their blue soldermask, it looks like paint. PCB quality is fine but I hate their customer service. They offer all of these additional services like machining and 3D printing. I had something that I didn't want to machine myself so I sent it to them. 9-11 days is now about a month with no answer, it'll be the last time I use them for anything.

JLCPCB on the other hand, I'm sure I've had at least 1000 PCBs made by. No idea what the previous poster is talking about, I've never seen an error on a PCB. I've made plenty of mistakes, but the only problem I've ever had with JLCPCB was ordering 10 4-layer PCBs sometimes they mess one up and they email me about it and I'll receive 9 with a coupon. I always order 10, cuz it's barely more expensive than 5 even if I only need 1. And I even sometimes order PCBs that are designed pretty sketchy just to test something as I hate sitting there messing with breadboards. I have some boards used in industrial machines that have been there for several problems
Several years ago, well before the pandemic, I saw quality projects on some JLCPCB boards. I'm pretty sure they've improved since then.
Several years ago, well before the pandemic, I saw quality projects on some JLCPCB boards. I'm pretty sure they've improved since then.
I've been using them for years, I haven't seen an increase in quality because it's always been good. I assemble my own PCBs and don't get into BGA type stuff but I do some 4layer boards. At some point I'll finish my PnP machine, but that's a different story. I've done several Teensy carriers as well. Just some random pics, most of these are old revs but their just pics I have on this PC at the moment....

I've got a whole lot more than that, they're not all winners but that's not JLCPCB's fault. I saw a video on Youtube of JLCPCB's factory a week or 2 ago, it was a long video. You think "Chinese" manufacturer, it must be a sweatshop or, lol. All I kept thinking was we can't have factories like this in the US that produce things this quickly, this cheaply, and be this just doesn't exist in this country, sadly.
I'm using JLCPCB on most of my project, because China is close to me... much cheaper shipping available.
I believe PCBway is also located in China, they provide near same service (PCB/assemble/3DP/CNC....) not sure how they are related but pricing seems a lot cheaper on JLCPCB in many ways.

My projects didn't include 4-layer or very complicated, components not smaller than 2020LEDs. PCB they all good unless I have my own copper design error unchecked.
I do found that each batch, or even boards in same batch, could have dimension errors probably due to different machine or drills.
That error sometimes I would need extra cutting with my knife to fit, because I have 3D PCB designs (slotting each other). I believe most case the error won't become a problem in any other project.