I would just as soon do away with the default list. I never post in non-default subs, and would never make a new sub, because why would I intentionally put my post somewhere with limited visibility. The users who are only using the subscribed tab will also never even see a new sub. You incentivize every user capable of thinking rationally (a minority here thankfully) to limit their posts strictly to the default list, this is not a sensible system and will not create a desirable outcome

Default should be subscribed to everything, with the list managed by removal. Every new sub should be auto-added to everyones list until they decline the content by removing it, or select some option not to auto-subscribe to new subs, but it should be the default.

The other way would be to make default subscribe to nothing and add them manually like reddit, but this doesn't seem like the best system for a site with few users and where google isnt driving interested people to the relevant sub in the search results.

For this site, the best way to manage the subs is everyone is subbed to everything unless they unsubscribe. You could still let people 'uncheck all' and add only a few if they really wanted. The other options are more problematic than any pitfalls of this system given our very small userbase, and the utter stupidity of the default list crippling the potential reach of any non-default subs and creating the incentive to post everything to only default subs like 'whatever'