Background: My Husband Is Into A Lot Of Different Conspiracy

Background: My husband is into a lot of different conspiracy theories and likes to share what he's currently studying...lately it's flat earth. I try to be respectful but the conversation goes on and on and turns into a lecture. When we are having a casual conversation or watching a TV show he looks for opportunities to bring his beliefs into play. For example, "look how the are trying to brain wash us by placing a round earth globe in that TV scene." When I share how I'm feeling and that I don't want to have our time dictated by his beliefs I get accused of being critical. I'm not trying to shut him out but I get so tired of the lectures and him creating opportunities to bring his beliefs back to the center of our evening. How can I set a healthy boundary without being "shut down" by him saying I'm just being critical? After I share how I feel he pouts like I've done something to disrespect him rather than me just sharing my feelings. Then I feel guilty and cave. Trying not to cave that's why I asking this question. Things are wonderful as long as I agree with him or don't call anything into question. We've been married for 30 years and I'm so tired of this game.