Consent is used when processing is not required to provide the service, no other basis applies, the processor still wants the data anyway, and the subject agrees. Legitimate interest applies when data isn't needed to provide the service but is necessary for related business functions (like fraud detection) not covered by another basis.
No basis except 6(1)(a) requires the subject's consent because the rest all cover processing required for the service to work.
https://gdpr-info.eu/art-6-gdpr/
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Edit: I read "They're not" as "the bases aren't separate." On rereading, it could also be the interests aren't 6(1)(f) legitimate. Which, agreed, they're usually not. I'll leave this up in case it's useful to someone.
That was the point. Then to force your users to consent adds insult to injury and is in contravention to the GDPR.