Hi,
Reboot is must if doing upgrade from SEP 11.x to SEP 12.1.
Supported upgrade path is common for both i.e. SEPM and SEP clients
SEP 11 RU4 --> SEP 12.1 RU1 --> SEP 12.1 RU1 MP1
If not facing any issue then I would suggest to postpone the upgrade plan.
SEP 12.1 RU2 may release in upcoming months, probably after RU2 release you can directly upgrade from SEP 11 RU4 to SEP 12.1 RU2.(But not sure also at this point)