I always use a memory disk inserted with mdsetimage. Only some small edits to kernel configuration files.
https://ftp.netbsd.org/pub/NetBSD/NetBSD-current/src/sys/arc...
I always boot from USB stick where possible. On RPi I boot from SD card.
I make custom userlands for the memory disk using crunchgen and makefs. Often I'll add a static tinysshd.
In the beginning, many years ago, I learned how to "bootstrap" a "diskless" system comprising what I need day-to-day in a userland without having to compile anything. Never seen this approach in any published tutorial. Basically, I used an INSTALL kernel and substituted the /rescue crunched binary for the limited "install" one.
I will either put a "complete" userland, i.e., with compiler, etc., e.g., as a cloop2 compressed image file on the USB stick, then mount it after boot, or I will chroot to an external drive containing a complete userland.
Where possible I rely solely on the computer's RAM as writable "disk" space.
There is never any data stored on the USB stick or SD card other than configuration files. The OS is read-only, immutable. The stick or card can be removed after boot.