What are you adding gradually, specifically? Like, a concrete example that names a namespace you may want to use. I'm trying to figure out what problems a half sandbox solves, and a vague "I just want to enable some capabilities" doesn't help here.
The sandboxing and mount-related ones are implemented with namespaces, and the idea with them is to not make any of them mandatory so they can be slowly added to system services. That way you can get some of the benefits without needing to build a full rootfs/container for the service. I am not sure how any of those would be done with jails because jails require you to create a chroot and network interface, whereas in Linux the mount and network namespaces are just optional namespaces and you can still use the other namespaces without using them.
They don't: you may chroot to /, share the host's network interface, or disable networking.
trasz@v3:~ % doas jail / foo 127.0.0.1 /bin/sh
# ps aux
USER PID %CPU %MEM VSZ RSS TT STAT STARTED TIME COMMAND
root 37975 0,0 0,0 13500 3056 3 SJ 09:11 0:00,01 /bin/sh
root 37976 0,0 0,0 13624 2776 3 R+J 09:11 0:00,00 ps aux