Your result
Possible
Nothing is available on an engagement — Brazilian law gives the residence and the shortened naturalisation to a spouse or a partner in a stable union, and an intention to marry is neither. But you are close to a route that works, and there are two ways onto it.
The obvious one is to marry. The less obvious one is that you may already qualify without marrying: a stable union (união estável) counts for both the residence permit and the one-year naturalisation, and it does not require a ceremony. What it requires is proof — ideally a union registered before a notary, or otherwise the documentary trail of a shared life, such as a shared address, joint accounts or children in common.
That second point matters for couples who have been together for years and were not planning a wedding: you may be able to start now rather than after an event.
Where you marry also affects the paperwork. A marriage celebrated outside Brazil is usually worth transcribing into the Brazilian registry, because that is the version Brazilian offices work from.
Tell us how long you have been together, whether you live at the same address, and where you are planning to marry, and we will tell you whether a stable union is already provable or whether the marriage is the faster path.