Sunday Ticket is available as a standalone purchase; you do not need the YouTube TV base plan for it.
However, keep in mind that Sunday Ticket is just for the regular season. It does not include playoffs. And during the regular season, it only includes out-of-market Sunday afternoon games (at 1 pm ET and 4 pm/4:30 pm ET). Games that air on your local FOX or CBS station are not included, nor are Sunday Night Football on NBC, Monday Night Football on ESPN, or any NFL Network exclusive games. For that you'd need the YouTube TV base plan, or a plan with another provider. Thursday Night Prime Video games are also not included.