Any data you collect that you do not unambiguously need to provide the service would be an appeal for "legitimate interest" as a legal basis for collecting it. There are a number of things GDPR writes about it and of course you cannot be sure how this will play out in practice, but the main points are:
* it must be reasonable from the user's perspective
* there must be alternative; you cannot achieve the goal (your "legitimate interest") without it
* it must be balanced with the rights of the user, and not infringe on their freedom or fundamental rights
* if your "legitimate interest" is direct marketing, the user can always object, and you are required to actively inform the user of this right
See also [1]