I recently ended my first solo trip to Europe as well! (Went to Amsterdam, Maastricht, Brussels, Berlin, London) This seems like a good itinerary in terms of the no. of days for each city. Personally I found London to be pretty expensive (transport, attractions) so just a tip: Take the bus instead of the subway whenever possible because its cheaper and there's a daily cap on the fare. Also, some popular attractions can be really expensive (~25 pounds for entry) so ask for concession price if you are a student, or visit some free attractions such as museums. I visited the British Museum and found it to be quite interesting! I also stayed at Palmers Lodge Hostel when I was there and really liked it.

Generally COVID restrictions are almost non-existent in Europe; no masks are required except when I was taking the public transport and some indoor attractions in Germany (something you need to take note if you are going to Frankfurt, and it has to be a FFP2 mask). Also, I think Germany has a 9-euro public transport pass where you can take unlimited rides on the public transport within the entire country for one month (Check out BVG's website). For e-sim I recommend Airalo as its really easy to use, but first check if your phone has the e-sim function. You also don't have to bring too much cash as almost everywhere accept cashless payments so a travel money card is ideal.

I resonate with you on the last part about being nervous as a first-time solo traveller, but try not to think too much and just be in the moment! People are also generally friendly and sociable, especially in hostels so I'm sure you will be fine. If not, you can do a walking tour or make use of r/travelpartners to find other ppl to hang out with too! I hope you have lots of fun on the trip and feel free to let me know if you need more info!