AWS DMS Service and Limitation

I would suggest migrating MSSQL on prem server to RDS with latest versions as long as you dont have any app compatibility issues. this way you are not on eol or approaching eol versions.

https://docs.microsoft.com/en-us/sql/sql-server/end-of-support/sql-server-end-of-support-overview?view=sql-server-ver16#:~:text=across%20multiple%20regions.-,Lifecycle%20dates,-The%20following%20table

DMS doesnt have limitation on migrating based on onprem db size. Please note required timing for data migration is combination of network/source db size/table lob distribution thus i would suggest start testing in nonprod environment and that give numbers to analyse