What about a new mixed-feed which merges the Subscribed, Local and All feed based on probabilities customable by the user e.g. 10/40/50 or 0/20/80?
This would push users toward local posts (especially on smaller instances) while still supporting a better distribution of user engagement over the threadiverse. And if you don’t trust that it was implemented correctly – you can simply check it by counting the amounts of posts against your chosen probabilities.
Meaning that it will store your preference in a per user file and learn from your behavior. Tracking your every like and every comment, measuring every word you posted against every tagged key words available and serving you the most probable posts that matches the most with your profile.
And of course this would be open to the public given the nature of the fediverse. And then advertisers would just waltz in and pluck it from the records.