Short answer — do not tell them until you have an offer in hand. There is very little upside and a lot of risk & especially on a small team. Once a boss believes you are leaving, you can be pulled off key projects, excluded from planning, or quietly targeted for the next layoff.
Even in good cultures, managers protect delivery first and people second. Transparency only helps if you truly trust your boss and the company has a track record of fixing issues fast, and even then a counteroffer usually only delays the exit. Keep your one on ones focused on the work and your growth, not your job search.
Answer the quarter question with specifics like, "Next quarter I plan to deliver Project X, reduce tickets in Y by 30 percent, and I would like to take on more ownership in area Z. Can we map a path to that and talk about compensation and title expectations?" If they probe about interviewing, use a neutral line like, "I am thinking about my longer term path and market value, but my focus is hitting our Q3 goals," and shift back to performance and development. Schedule interviews outside work or use PTO, call them personal appointments, and use your own phone and email for all recruiting touches. When you do get an offer, give clear notice, propose a handoff plan, and leave with relationships intact.