To stop a service A with a timer, you can create a service B of type
oneshot that it will be in conflict with, then use a timer to start the service B.
If a unit has a Conflicts= setting on another unit, starting the former will stop the latter and vice versa. (source)
Description=B service description
The following will stop service A after 10 seconds.
systemctl start A.service
systemctl start B.timer