I considered making an alt account for this but couldn't find a server that let me do so without waiting for approval so whatever.

I've been in a very similar situation. Good friend hit a rough patch, moved in with me, a relationship developed but at the same time I knew I'd want to see other people.

The most important thing here is definitely communication. You say you don't know how they would take it so I assume you've never talked about this. So I urge you to talk to them as soon as possible. If you can, do it today. Don't fall into the trap of waiting too long. I've learned that the hard way in a previous relationship where my partner and I had very different plans for the future and, not wanting to upset them, I waited for a good time to bring it up. Guess what, having something so important in the back of your head will strain your relationship to a point where there is no good time to bring it up.

If you don't want to scare them, start with hypotheticals like "how would you feel if I decided to see other people?" then work your way up. Be gentle but be honest. Tell them that wanting to see other people doesn't mean you love them less or that they are not enough. For me, comparing it to other things I love like a favorite food or hobby worked well. Let's say I love cheeseburgers to the point that my friends call me the cheeseburger guy. No matter how much I love cheeseburgers, sometimes I need something else. Eating a pizza doesn't mean I don't love cheeseburgers anymore and I would never want to stop eating cheeseburgers. Just make sure they know you're not comparing them to a cheeseburger unless they're into that.

Find out what they want. Best case, they are in the exact same situation as you and are just as afraid to tell you they want to see someone. Not that likely but possible. Maybe they are okay with you seeing someone but don't want anyone else for themselves. Maybe they want to change your relationship in a way where both of you are happy being the only ones for each other. And maybe it turns out that your plans just aren't compatible. Be prepared for that. Think about if you'd rather give up on your plans or on your friend. Both is fine, it's your life. Not talking about your wishes doesn't make you more compatible, it just builds tension until things go spectacularly wrong.

And most importantly, respect their wishes. If they tell you that they don't want you to date other people, that's absolutely valid. Don't try to persuade them and for the love of everything, don't do it behind their back. Either don't date other people or make sure to end the kind of relationship you have at the moment. Hopefully you can stay friends.

Finally, about that other thing you mention, other potential partners not accepting that you have someone you share your home and even bed with. Don't worry too much about it. When I started dating again, I noticed that there are surprisingly many people out there who are totally fine with being open, poly or whatever you want to call it. Especially on dating apps. I don't know about Tinder but OkCupid even lets you filter for that. Just be honest about it.

Edit: slightly updated some wording after I read some of your replies which made the kind of your current relationship a bit clearer. The original message still stands.