Hi Wiebke,
I’ve been called a cat whisperer. I’m not sure I agree, but I do have a good rapport with most cats. It’s sounds like you are on the right track. Hissy may never want to come inside. That’s ok. Keep being calm, speak softly and be friendly without pushing. Seeing you with the indoor cat will give her confidence that you are ok. It can take a long time. I had a formerly feral cat, the best cat ever, but it took a year for her to trust me.
Good luck!
You really have to make yourself interesting to the cat so that they want to come up to you. Learn to read their body language so that you can withdraw the second they look like they want to leave and then just maintain that position while talking them and telling them what a gorgeous puss-cat they are. Tickle a piece of grass on the ground so they want to investigate. Essentially let them come to you. I have had a lot of success with cats who never go near anyone with these simple methods.