Want to take gf to Germany and propose. American guy here, could use some help/advice

Hey everyone, I'm a 33 year old American dude wanting to take his gf to Europe this year and propose. I have been to both Germany and Switzerland once like 12 years ago and loved them both. Germany was very fun and Switzerland was very beautiful. I know the countries are close and we could do them both, however I've been researching a little and am trying to find a good time to find festivals, celebrations, etc. Total time spent in Europe will likely be 7-10 days.

Not really any particular time of year, just not September (already have a trip planned). Octoberfest would be really cool, although I'm sure the crowds are awful. Also read about the Rhine in Flames and that sounds really great, particularly in Koblenz since that's supposedly (according to to Google) on August 9th and Swiss National Day is August 1st, and we could hit Basel for that and then go up to Frankfurt a couple days later and stay until the 9th before leaving the next day or something.

I have never planned an international trip before and only recently started researching. I don't know much about traveling in Europe so If there is anything I should know, if you guys have any cool ideas or suggestions, or if I should ask this in a different subreddit please let me know. Thank you 💙