Though privacy is a concern, Bambu's LAN mode seems to adequately address it (for now). The real concern with Bambu is the lack of freedom to go outside the Bambu ecosystem. You're locked into a walled garden and subject to the whims of the company. They're reasonably benevolent now, but lawsuits or product bans in NY or CA could see them singing a different tune in a not-hard-to-imagine dystopian future.
By contrast, QIDI is happy to let you own the printer that you bought. You can firewall it and it still works (without special LAN-mode shenanigans). You can use any slicer and it still works (without having to sneakernet files on flash media). You can load alternate firmware (even firmware you built yourself) and it still works (assuming you do it right). If those things are important to you, then they tend to make the QIDI a better overall value.
It's more than just scaling the bolt. Heavier bolt means heavier recoil springs. Heavier recoil springs means larger/stronger receiver. Magwell and magazines also have to change for each caliber. Magazine design is one of the most difficult parts of creating a self-loading firearm. It's easy to make one that functions; it's hard to make one that functions reliably (even when using an existing design as a reference). You can "cheat" and rely on commercial mags, but even then you'll usually have to tune the feed geometry since the behavior of a .45ACP cartridge is going to be radically different than something like a 5.7.
There is a 9mm version of the Decker 380 in development right now and despite the cartridge itself being nearly identical just 2mm longer, nearly everything except the fire control system has had to change to some degree.