sheardude
I'm in the same exact situation and I'm in the process of making it a LTR.I think if you follow my steps you might get the same results. If anyone else has input into my advice please give it. We're here to help each other. I'm not a pua but I want to help you so you can enjoy the same results I'm having.
First and most importantly you need to get multiple female "options". Even if you want to marry her this is important on two levels. First you will be able to compare what you have with others and get a realistic portrayal of her. The same way you drive many cars before buying, the same concept applies. Second she will not see you as needy. There is no danger of you becoming a stalker and it puts buying pressure on her. Think of an auction, the object gains more value if more people are interested.
Second Don't worry about building comfort. You already have that. She has known you for many years and feels comfortable sleeping besides you. You LITERALLY can't build more comfort than that. The more comfort you build here on out will work against you. It will only put you deeper into the friend zone. If you continue to build comfort she will see you as needy she will think you'll always be on the sidelines.
NEVER repeat NEVER go on a formal "date" until everything is official. I text a few of my options and my target to a cafe for lunch while I study, she's there just for company. I do this very spur of the moment. It's framed as if she's entertaining you. Now if your option and target show up together it's jack pot. They both KNOW you're high value. Because its last minute she can opt in or out and it's ok because it's so last minute, plus because it's so casual she doesn't feel like she's on a serious "date" which takes sexual connotations shes not ready for.
isolate the target and bounce multiple places and push for a sexual/flirtatious vibe. Be reasonably aggressive e.g hint at romance, her beauty and sex. If she has even the SMALLEST amount of attraction to you she will give a positive response.
Once you escalate the vibe to reasonably flirtatious/sexual you should go for a K-close. from there she'll be your girlfriend. All the comfort you've already have plus personal history and the romantic love you guys will have will definitely lead to a healthy relationship
I sincerely wish you the best.
Side note. My personal game is a mix of Stealth seduction and David X's direct approach. researching their techniques before using my advice will help you with all the many middle steps I didn't list. This shouldn't be hard ,you two did sleep side by side and she DID reach out to hold your hand
CHAT