Hi @Irene !
Welcome to the wonderful world of bike packing! I am a huge fan of on the bike bags rather than using racks and panniers. The Link Seat Pack as well as the Ortlieb and Revelate Designs are all great options. I've used the Ortlieb Seat Pack for the past 3 years or so. The material is heavy duty and certainly keeps my gear protected. I like that it also has an air purge valve to let me squeeze any extra air out, allowing for a compact air and water tight pack. I’m on a 54cm bike, and I don’t have my seat tube pulled up very high. The large, which has two velco straps, just barely fits. The medium pack only has one velco strap and it would probably be a better fit for your bike.
One thing to note with the Link is that it is considered water resistant and the fabric is treated with DWR (durable water repellent). This will be fine for tire spray or light rain, but if you're planning on being out in excessively wet conditions, you may find that eventually water may permeate through the fabric on this pack. The Link would likely be a good fit since it also just has one velco attachment strap at the seat tube, and has the bungee strap on the outside for quick access storage.
I haven't personally used the Revelate Designs packs, but I know they are also quite popular. One advantage of their Terrapin pack line is that they have a couple additional attachment straps on the outside of their pack if you want to store a jacket or other equipment for quick access. Like the Ortlieb, they also have the air purge valve, and a lot of people are a fan of the two-piece system. The frame of the bag can stay mounted on your bike, and the dry bag with all of your gear inside it can be removed for easier access as well as easier packing and unpacking when you’re at your camp site.
I hope this helps. There are plenty of wonderful seat packs that you can use for bike packing, and I think you'll be happy with any of these bag options!