I believe the correct command for systemctl to reboot is:
systemctl reboot
in fact I just did this on two of my Rocky machines, both 8 and 9. As for isolate command, I believe you normally use that to change between different runlevels, like you used to use runlevel 3 or runlevel 5 to go from console to graphical. Using the commands to list targets:
[root@rocky8 ~]# systemctl --type=target
UNIT LOAD ACTIVE SUB DESCRIPTION
basic.target loaded active active Basic System
cryptsetup.target loaded active active Local Encrypted Volumes
getty.target loaded active active Login Prompts
local-fs-pre.target loaded active active Local File Systems (Pre)
local-fs.target loaded active active Local File Systems
multi-user.target loaded active active Multi-User System
network-online.target loaded active active Network is Online
network-pre.target loaded active active Network (Pre)
network.target loaded active active Network
paths.target loaded active active Paths
remote-fs.target loaded active active Remote File Systems
slices.target loaded active active Slices
sockets.target loaded active active Sockets
sshd-keygen.target loaded active active sshd-keygen.target
swap.target loaded active active Swap
sysinit.target loaded active active System Initialization
timers.target loaded active active Timers
doesn’t show reboot.target, although that can be found on the system. It could well be that reboot.target isn’t supposed to be used in that way, and is why it doesn’t work. To be honest, it’s easier and quicker to just type reboot, or even shutdown -r now. Even systemctl reboot is shorter than using the isolate command.