Regarding points 1 and 2, I’d try not to see this as a binary. When you launch, there will probably be some people who like it, and some who don’t, and that’s completely fine! See this as an opportunity to get feedback on how to make the app even better, rather than a measure of success or failure.
There is so much you can do after launch to keep iterating and improving from user feedback. The best thing you can do is continue talking to users & give them opportunities to provide honest feedback regularly.
For point 3, this really depends on who your target user is. Try to target where those people “hang out” online, and use messaging that focuses on the problem that your app solves or the specific value it would add to their lives. Again, talking to your users is key for developing effective marketing.
If you’re looking for more advice on talking to users and collecting feedback (or even want to think about outsourcing some of that), I do this full-time so I’m happy to chat more.
Not sure about your budget, but there are some online recruiting tools that can help with this - Respondent.io and Zintro are two of the more affordable ones.
Otherwise I agree with the comments about finding events or communities that your target users might be a part of! Can you find any Facebook groups, Slack communities, etc. that might be relevant? In-person events like career fairs are great too.
Another option for validating your idea would be to run paid ads highlighting your value proposition (again, think about your audience when choosing the channel for this), and to see if they are in fact converting. Even if your website isn’t ready, you can direct people toward a landing page highlighting the website’s value, and ask them to join a waitlist / get notified when it launches. If you see strong conversion, that’s a great indication that your product is resonating with the audience you’re targeting (and as a bonus, you can capture their contact info to reach out for follow-up conversations). If the ads don’t convert, it’s an indication that you might need to do more discovery work around your target audience’s needs & how to align your product with them.