Is this a bad way to propose?

For context me and my partner (both nonbinary) are both undergrads in college, and have been dating for around 6 months. A few days ago the topic of marriage got brought up, and they mentioned that because they asked me out, I should be the one to propose. We are both fairly young, so I know nothing is certain, but they’re such a good person and I really want to see them grow and be apart of their life. I don’t want to propose right now (but I want to eventually), but my idea on how I want to do so needed me to do some prep work.

Anyways, a day or so ago I wrote them a short letter, alongside the date and how long we have been dating. Nothing too long, pretty much just a very personal way of describing how I feel towards them. At the end I asked them to marry me. I’ll probably give this to them in a few years, maybe at like the 3-4 year mark (this is mentioned in the letter), but I want them to know that how long I’ve loved them. The idea being that they’ll read it and afterwards I’ll say something stupid and then they’ll call me an idiot and say yes.

Is this creepy or sweet? I think they’d prefer the simplicity of it (they’d hate something public or grand), and I have a few years to make sure of this and any other details, but if the idea is too stalker-ish than I’ll go back to the drawing board.

Any advice or concerns would be greatly appreciated 🙏🙏🙏

tl;dr - I wrote them a letter saying the date and that I love them an will give it to them in a few years.