I think I have a solution to your strap issue where you can keep using the regular webbing.

For the top portion you highlighted, remove the strap from the existing buckle and D ring on the bag itself. Then cut off the plastic D ring from the bag (cut the plastic, not the strap) and replace it with one of these. Then you can just flip the webbing around so the velcro end goes through the cobra buckle instead of the D ring, and you can feed the sewn closed loop through the opening in the new D ring.

I had to do some similar tweaking when I swapped the buckle on my Rhake with a fidlock.