Yes, it is mandatory to take registration if you are receiving any service covered under Section 9(3) on which RCM is applicable.
Legal Services and Manpower services (Received from non-corporate entities) are among the services covered u/s 9(3).
Further, please note that RCM u/s 9(4) is suspended and RCM is not applicable on purchase from unregistered vendors anymore. Therefore due to this clause, you need not take GST registration.
Trust this meets your understanding.
Thanks.