If you want to do high-end stuff then why are you buying a laptop? If not, look into business-line machines from yesteryear, often companies get rid of them and then they're sold by "refurbishers" on eBay. They're not high end machines usually either but the build quality is completely different from the consumer crap. Mostly they usually come with appropriate amounts of RAM with good speed rating with the maximum amount of channels the system supports which is never mentioned anywhere but makes a huge difference in perceived performance, especially for iGPUs. (This is the real reason why cheap machines often are crappy - very slow, single channel RAM)
You actually usually also get a very nice screen and decent wlan/drive in mpcie slots, often with a third option slot that's usually intended for a 4G modem. (If there's no bios blacklist you can stick whatever in there) Stuff on them usually just works and they're well put together machines in general that are meant to be opened (as opposed to glued together) and the build quality is something you can feel. If you shop around a little and are patient you can get a decent used machine for the price of a new bottom-of-the-barrel craptop. If you're stingy and handy, you can look for machines with broken touchscreens (these business machines often have wacom stylus/touch screen combos that get angrily stabbed to death by fat-fingered business boomers) or case cracks and see on aliexpress if spare part sourcing is easy and then do the math if it'd be worth it to repair the machine yourself. Often it might be actually preferable to get a machine in known, broken condition and make it "new" again with spare parts as opposed to a machine where the seller insists all is fine but you don't really know what you'll be getting. The former are usually a lot cheaper and for the bussiness-lines you actually often can download service manuals where it's explained how to do the replacements.
I have a Threadripper Pro workstation system. It works great and it is nearly infinitely expandable. The problem is I need a capable computer that I can grab on the way out the door, because sometimes I am away from home.
I've been playing the off-lease business class laptop game since at least 2008. Most of my previous laptops have been used ThinkPads from eBay. I've had a few consumer laptops too. Dell used to make some really good ones in the Pentium M and Core 2 eras.
Business class laptops are trash these days for all the reasons I listed previously. I understand it's
even worse in the consumer space. I actually bought a brand new, top of the line business class laptop in 2019 and it was so horrible I sent it back after a week. I bought one in 2023 and it lasted a little over a month and now it won't power on. Ivy Bridge performance isn't good enough for me anymore. I need something new.
Part of the problem is the race to the bottom in terms of cost, and another part of the problem is the "thin and shitty" trend that everybody is copying from Apple. Now Apple laptops have always been shit. Anyone who tells you otherwise never owned a TiBook or an AlBook or an aluminum MacBook Pro or a chicklet keyboard MacBook or any number of other shit designs Apple has put out. But up until the past few years, we had choice. We could go and buy a Dell or Lenovo or even a Toshiba in your choice of thin and shit or thick and capable. Nowadays your choices are, thin and shit, or a workstation that looks like a gaming laptop and has less expandability than the model from last year despite still being a brick.
Now about the wifi, modem, and M.2 slots. The reason wifi whitelists exist is because of FCC regulations. Newer laptops are certified in all combinations of laptop model to wifi card model offered from the factory. Those wifi card PCI IDs are whitelisted and everything else is disallowed. In the early days of whitelists, the system firmware (old BIOS, new EFI) could easily be hacked to remove the restriction. Newer machines, especially these off lease business class laptops you speak of, are increasingly more difficult to hack and many are impossible. The newest two generations of Intel laptops build half of the wifi chipset directly into the PCH and the other half is soldered to the moterboard, so this is moot going forward.
Sometimes the (cellular LTE) modem slot will support wifi cards, sometimes it won't. Sometimes it'll take an SSD, sometimes it won't. There were 2 or 3 generations of Intel laptops that would take an mSATA SSD in their Mini PCIe modem slot. In fact, I have the guts of a 2TB Samsung T5 Portable SSD stuck in the modem slot of my current laptop. Some of the newer ones with NGFF slots might take SATA M.2, but more likely you'd need an NVMe drive with the correct (nonstandard for an SSD) keying, of which I think there are only 1 or 2 shit tier cacheless SSDs currently on the market. All of that assuming there is no whitelist on the modem slot, because some actually do.
Then there is the main storage M.2 slot. On a sufficiently new laptop, you usually you get one or two of these. On some of these business class laptops, those are also whitelisted, but in a different way. They will refuse to boot if the capacity is detected above an arbritrary limit. A lot of Lenovo laptops won't take SSDs bigger than 2TB for example.
Almost all of the smaller laptops have had soldered RAM, or soldered RAM plus one SO-DIMM slot for years now. Long gone are the days where you can pick up a cheap used laptop with low specs and very conveniently quadruple the RAM. Since most new laptops are thin and shitty, this is rapidly spreading to laptops of nearly all sizes. On top of that, it's hard to find one that will take 64GB of RAM, and a lot won't even do 32GB. With the ever increasing resource consumtion of things like operating systems and literally every new program, that's a problem. Remember, your current laptop is worse than your old one, and your next laptop will be even worse than your current one. So it's important to go as big and best as you possibly can, now.