Hi,
I have tested many rack bags for strida.
Out of which I have tested I think that pannier backpack are the most convenient for everyday life.
Good example is Oak25 pannier backpack
Why?
Easy to carry around
Fast and easy to put on and take it out of the bike.
Doesn’t affect folding
I have tested Toppeak rack bag with Velcro fasteners beneath it . This bag has good cpacty but it is really inconvenient to put and take it out as the Velcro run under the bag. Maybe good if you are not planning to take it out from the bike like one-day trip. It is possible to use DIY to make it faster to fasten it to bike.