If you have not yet, it is worth reading the previous part of this series for the best context.
Flutter Web: Should I use it? (Part 2 — Performance)
While SEO with its scrapper and performance factors are definite considerations to take, they are not the only things you should consider when deciding whether or not to use Flutter.
While Web is probably the weakest side of Flutter at this point, that is very likely to change in the future, and especially if you are working on a larger or a longer-term project, what is true today is almost certain to not be so, or at least to a much greater extent further down the line, and possibly even a non-issue by the time the project is ready.
As I mentioned in the first post, you/your team are the only ones who can truly determine whether or not choosing Flutter is a good choice, you are the only one(s) in a position to fully understand the potential costs of any framework you choose for your project.