How can one best seek out collaborators in obscure topics as a young researcher?
As mentioned by aparente001 in the comments, the best way to start collaborations as a beginning PhD student is through your supervisor.

Your supervisor is hopefully a well-established researcher who's made some decent contributions to the field. People working on close areas of research will likely know their name and what kind of things they work on, and consequently the sort of things their students are likely to be doing.

Your supervisor may bring up the idea of collaboration without you asking, or you could approach them with the idea you have and, if it has merit, they might suggest names of people working on similar areas whose work you could read up on and get in touch with.

In this day and age, it's easy to start a conversation through email, and your second point about corresponding with potential collaborators and asking questions about their work is a good approach, but make it clear who you are,  what you're working on and why you're interested in their work from the outset.

If you're serious about starting collaborations with people completely outside your network (or your supervisor's network), the best way to do it is face to face, at conferences, meetings or summer schools. The latter are particularly useful for PhD students, as the attendees are also going to be PhD students or post-docs, who will definitely have more time and incentive to start new collaborations than senior academics will.

Lastly, don't worry too much! People are aware that new PhDs are still finding their feet in academia, and as long as you are professional in your enquiries, I don't think you'll go far wrong. It's certainly not bed etiquette to want to work with different people and broaden your research horizons.