currently there is only way to subscribe to a subscription list via landing page.
Subscription list is a powerful feature that AJO offer but it's hindered by limiting it's ability only for landing page use cases.
My suggestion it to enable the update activity in journey to use subscription list to subscribe from journey and also to have a history of tracking when a profile unsubscribed from list and when a profile subscribed, the trail will help us debugging and have a tracking details to help on prod issues like accidental un subscriptions from landing pages.
https://experienceleague.adobe.com/en/docs/journey-optimizer/using/landing-pages/subscription-list
