I've owned 2 Gaggia Classics and a Simona Top DE. The Gaggias are awesome little machines capable of producing excellent coffee. A PID isn't absolutely necessary, but it certainly makes them easier to use. And the other mods you've done along with an adjusted OPV make for a very capable machine.
The Simona Top is definitely a step up. While the Gaggia has a commercial size group it's not really a commercial group as such whereas the Simona Top very much is. It's still small as far as HX machines go, but it has some very clever design features such as an actively heated group head. The only mods I'd suggest you consider would be to insulate the boiler and replace the steam wand. Mine had an OPV, but the seller wasn't sure if it was factory fitted or not. If not you'll want one of them too.
The single biggest improvement in flavour of coffee that I noticed through all the equipment I've owned was when I finally took the plunge and stepped up to a flat burr grinder from a small conical burr. So while your Breville Smart Grinder is more than adequate, something like a Compak K3 will provide a noticeable improvement in taste.
Enjoy your new machine, look forward to seeing some photos at some stage.