I like the suggestion about mounting at the lids.

Yes heavy, but no more so than a set of aluminum panniers.

Soft bags, much lighter. Good for many applications, especially shorter trips and yes carry most things. But on longer trips tradeoff is:

1. Slightly improved security for hard bags which depending on your ride you may or may not need. For example, popped into the monastery in Kharkorin in Mongolia and left hard bags locked outside in front of vendor area. Felt fairly secure that someone wouldn't sidle up to the bags unnoticed and go through noticable effort required to break in. Would not have felt secure leaving a simple soft bag where the the usual crowd milling around squeezing the levers could have gone through the bags without too many people noticing. Personal perception I agree, but I certainly would not have felt safe enough to leave a soft bag unattended, even with a lock.

2. Crashability: They simply bounce better. At low to moderate speed crashes they simply bounce and don't require any straightening. The attachment hardware or racks might depending on your system, but the good old soft bags will eventually get a little ripped and torn and require some repairs or extensive bungee cording. It can be done, and when you get to a town you can get them sewn up or a rack made to hold them but after a while they may not work as well they originally did. Of course, that is all moot if you never crash