I recently bought a 13 liters saddle bag. I used it during many trips this summer and I love it! The only problem I had is that when I stop riding I have to detach the bag from the bicycle and carry it with me to prevent thief to stole it. This operation last a couple of minutes but when you need to stop often, it's very annoying.
My girlfriend usually carry a backpack but during the summer it is very uncomfortable due to the back sweating a lot. So she wanted to buy a saddle bag. I told her that a luggage rack would have been a better choice.
I found this product: https://www.amazon.it/gp/product/B08FB1HNWH/ref=ppx_yo_dt_b_asin_title_o06_s00?ie=UTF8&psc=1. Cheap, good quality and light! It can carry a maximum of 50 kg (remember to mount every component because if not correctly installed it can carry only 25 kg)
The main advantage of the luggage rack is that you can carry a backpack and tie it to the rack using an elastic band. Tying and untying the backpack from the rack is a matter of seconds...
There is another big difference between the saddle bag and the luggage rack: stability.
When you are carrying heavy loads and you are on a rough road, then the saddle bag is always moving from left to right and up and down.
The luggage rack is much more stable even if you are heavy loaded.