The unsung hero here is probably the nice!nano. Such a good little board. I've made my own split keyboard around them and it just makes so many things I thought I'd have to think about Go Away.
The nice!nano gets the job done but it's really expensive for what's ultimately just an nRF52 dev board in the ubiquitous Pro Micro form factor. They're $25 a pop and you need two for a split board, while you can get generic nRF52 Pro Micros on AliExpress for about $3.50 each.
It's the same story with RP2040 Pro Micros for wired QMK builds, there's a huge "keyboard tax" if you get the ones marketed for that purpose.
It's more a convenience tax. The nano also has battery management onboard, which is one less thing to think about. But really at that price difference picking the nano over the bare pro micro has to save me less than an hour of design time and it's already paid for itself if I'm just building a keyboard for myself. If I was looking to manufacture something at any more than "it's just for me" scale it would be a different story but it's extremely well targeted at its niche.
Pretty much every nRF dev board has battery management built-in, including the $3 AliExpress ones. The chip is meant for ultra-low-power applications so including battery management is a no brainer.
I'm not sure why the 'pro micro' format is so popular. If you are already designing a PCB, why not plop an Atmega32u2 on it directly (assuming you dont need wireless)? You only need a crystal, two resistors, four caps and a usb connector.
It's just easier for people with less soldering experience, all the tricky SMD parts are already installed for you. Especially now that QMK has shifted from AVR to more powerful and readily available ARM chips like the RP2040, which are often only available in packages that are difficult to hand-solder. Plus designing a keyboard around the Pro Micro means you can use the same PCB for wired and wireless builds.
