Sage feels strong to me and is strong. Numerically, the healing values are good. A big factor on whether Sage feels weak or strong is if you are using the mitigation and healing % increase effectively - optimizing this aspect, and using Kardia + Soteria will make SGE's heals feel strong. Using them incorrectly will make SGE feel a lot weaker in comparison overtime.

After playing SGE for a while now in pre-Endwalker and Endwalker content, I think a big issue for people thinking Sage feels weak is also because they haven't changed the way they prioritize their healing compared to other healers. The priority on healing abilities really changes as Sage level up, but this is largely due to whether you have access to Rhizomata (an addersgall meaning you spend more healing abilities using the gauge first to avoid overcapping and losing MP), access to Holos (another mitigation, which means you can spread out your healing more), access to Kerakeia (Lv 78 trait) and access to Panhaima. The priority of healing on Sage really evolves as you get higher in level, and it matters a lot more than other healing jobs because SGE just has a lot more synergetic healing skills, so it requires more thought on your part to play Sage effectively.