Strapi and Sanity are excellent. They are also not always the answer. Signals that suggest you have outgrown off the shelf content tools.
We use off-the-shelf headless CMS tools for most projects. They are excellent, well-supported, and our editors love them. But there is a band of clients for whom they quietly become a tax.
The signals are familiar: editors maintain a parallel spreadsheet of "the real source of truth", a quarter of the schema is "do not touch" custom fields, and the tool ships changes that take a sprint to absorb.
When that combination shows up, a custom CMS becomes the cheaper option. The schema follows your editorial model exactly. The interface is one or two screens, not ten. There is no upgrade cycle. The team owns the code.
The trade is fewer features for more fit. A custom CMS does not give you twelve preview environments. It gives you one, the one you needed.