I'm probably in your target audience.
Capture: notion and twitter have been best, obsidian and regular markdown have been worst.
Notion is good because of how they support a calendar view where you can put documents in a day's cell, and then see a list view that's just a stack of those notes. I keep a daily diary or youarehere type doc, where I'll have checklists and notes on small things that don't merit changes to a dedicated page. There's arguably a "retrieval" breakdown in that I don't really go back through these to update them or collate them into bigger pages.
Twitter is good because it's low friction and I can just go off, which is fun, and because they have decent search, so I can quote-tweet a related thing and sort of thread the graph together. If you're talking about BASB you're probably familiar with this corner of twitter. visakanv etc. This method works well if you use it enough to be able to recall your other notes. I think there's something special about the twitter format here too: it discourages whole-page thoughts in favor of sequential pithy bits, which i think are easier to both link and recall.
Execution: I would like a chat frontend (signal/SMS/etc) where I can just talk to my projects, ask the status of things, get suggestions, etc. Push based, rather than pull based, execution.
Active project context: I've dropped todoist-like things since they're limited in what they can express, and notion/markdown can do todolists etc. I tend to have lists in markdown style that live in two places: my daily diary/todo docs, and the actual projects themselves. This is messy and it would be lovely if notion or similar had the concept of a "todo block" and could collate all of them into a single view where I could understand association, prune and dedupe, etc. Even better if there's an agent that does or suggests cleanup whenever a new block enters.
Larger projects will get docs of their own, lots of sprawl and notes etc, and then some formalization around a spec or something. I move these to an archive folder when I'm done with the notes and the final document is fleshed out, but I'd love an agent review that makes sure I'm not leaving things on the cutting board, and that I've handled all the todos etc in my notes pages.
I don't use bidirectional linking/tagging enough, but I really should, since I want to be able to coin keywords for particular concepts inline, and then be able to access their overview and see everything that mentions them in a graphlike way.
Calendar is definitely a much used component day to day. For planning, etc. But it's not a source of truth. Everything on a calendar should just be a proxy/link to a more robust doc.
Hard nos: My take on privacy policies for things like this is "show me your incentives and I'll show you your outcomes". That is to say, any company that can survive an attempt to profit from data fuckery will do so. Your data retention policy should include technically unambiguous red lines that are not to be crossed, and define specific per-user monetary payout in the event that a breach occurs, to include clauses that cause user payout to occur before eg preferred stockholders get liquidation preference and drain the possible payout pool. Routine third party audits of how user data is handled/retained/distributed etc. I recognize that this is a bit unhinged, but that's what signaling credibility looks like. A company says "we won't sell your data" and I say "or what" and there's hemming and hawing because nothing will happen to them. If the answer is "this company dies on the spot and our investors get completely fucked", now we can talk.
I think AI service pricing applies here: generally, if it seems neat I could be in for $20 easy, and if it's genuinely game changing, $200/mo is completely reasonable to ask.
re Migration cost: I expect to be able to get 100% of my data in a reasonable non-proprietary format. If that's some blend of markdown, json, sqlite, whatever, fine.
But the bottom line for me, where does my second brain break down the most? It doesn't talk back to me. I want it to understand what I've got going on, and my idiosyncracies. I want to present it with new information and have it be like "oh, this relates to X" or, periodically, to pop up with something like "I'm noticing this correlation / related idea in areas X, Y, Z... does that resonate? Is there something here?" Again, push vs pull. My second brain should be a proactive chatbot. "Noise" is so often thought about in terms of frequency, but it's really about insight quality. If my response to 80% of push notis is "damn, good call" then you can send one every 5 minutes.
I also hear no mention of one's personal life. I don't really make the distinction. It's all in there. I should be able to bitch to this chatbot about my manager, have it know about that background, and riff with me to navigate hard convos. I should be able to talk to it about side projects I have going on, and let it thread those into my calendar. Etc. Notion is already an adequate second brain for work. Nobody has yet built an adequate second brain for the home. My house, my relationship(s), my side projects, my own diarying and self reflection... these are the contents of my brain that matter.
Email in bio if you want to talk. I'm a design technologist and happy to riff / give feedback.