This is a difficult one - even the hardiest shrub contained in a pot is vulnerable to freezing during a hard winter - if the compost in the pot freezes for longer than a couple of days, the plant will be killed because its roots have been frozen. If you can't move the container to a sheltered spot during winter, it might be best to stick to temporary, bedding type planting, changing it summer and winter. On the other hand, you might get away with a permanent inhabitant for a while, since you managed 5 years without this happening to the Mahonia. In which case, so long as the shrub you choose is hardy, it can be anything really, there's no advantage in trying to find something even hardier, because the roots might freeze regardless. What you choose depends as much on whether the container is in sun or shade as much as anything.

On a different note - I sympathise - and empathise - with you having a dial-up system! Been there! Done that! Happily no longer any need to - Spain was dragged kicking and screaming into the 21st century and Broadband about 2 years ago!
I also had a large tub to fill and now have a Phyllis Bide rose doing great things up a trellis near the porch. It is semi-protected, being so near the casa, but it is a repeat flowerer and, as long as I keep it well fed and watered, promises to go on to greater things.