Groups of objects.
Just so that you understand where I am coming from, I am actually a total IT geek, 32 years professionally (i.e running consultancies, not a salaried employee); normally I provide the architecture and the database, and 6 or 10 or whatever developers produce the code. OG is my drawing (and other) tool, 20 or so full-blown wireframes (sites not pages), and an awful lot of drawing before wireframes became the norm.
From your post:
1 You have decided on groups too early in the project; the titles, labels, ribbons, fontsizes should be stable before you proliferate them too much. Yes, of course you need a few copied instances before they settle down but do not render the entire site or large part thereof until the standards have settled down. Otherwise you end up with your problem, and the correction is tedious no matter which tool you use. If you have to make changes to large numbers of proliferated objects, I would say that by definition, your standards are not mature; and not mature enough to proliferate that many objects.
2 Separately. Once the standards are set, and you have drawn the site or large part thereof, you may still have to change (eg) the level-2 headings on many or all grouped objects. Given that your diagram (the entire site, not one page) is stable, you have to have a different attitude. It is no long a mock up, it is a rendered diagram, close to sign-off (and if it is not, then by definition, you are at  ).
First do this: View/CustomiseToolBar. Throw the Un/Group toggle out, and drag the Gruop and Ungroup buttons into the TolBar.
Select all the objects (which are each a group) you have to change via Canvas:Selection Inspector or by shift-clicking items all over the place. Let's say each group consists of 5 different shapes/lines, and you chose 10 grouped objects out of 100 on the canvas; you should have 1 well selected in the Canvas:Selection Inspector. Watch the Canvas:Selection Inspector during this next step. Hit the Ungroup button. You should now have 5 wells selected in the Canvas:Selection Inspector. Choose the shape/line/ribbon you want to change and <go back to my previous post, probably  >.
Do not bother grouping the 5 objects, 10 times; it is a rendered canvas.
Grouping is for convenience, not for fixing in cement.
3 You might find this thread
useful. Suggest you read the whole thread.
However, if I ever make changes to that group, I’ll have to manually replace each instance with the newly edited one.
Certainly not. I think you can figure it out from here.