I think Earth is on an even worse basis per person than as countries or planet wise.
Given what we have seen on the selective genetic engineering in this and the selective breeding just from base labour any of thier laburers would trounce any earthling (just on a manual labour planet) and it would continue up the labour chain any factory worker has 100,000 years of ohs and best practice knowledge behind them for any built world and any advanced world has had 100,000 years of progression from basic smart phone to implants.
TLDR
earth is the underdog in every respect regarding production.
It's funny you mention this because I'm debating on how to approach that when the SG teams do run into worlds a century or so ahead of them.
Because I could see some of those worlds abandoning such tech due to the social issues such things cause, or having a much more "living gods" take on the System Lords or see them as benefactors who are owed allegiance because they rebuilt the local galactic group etc...Which in turn allows them to have avoided all the social decay due to having an anchored society.
The sheer age of tradition is going to depend. Lotar worlds have 40 or so thousand years. The ones with Serrakins and other species will range from twice that to longer even than the Goa'uld themselves as many of the races that were clients of the Ori joined the Empire.
Granted most of them had been smashed back to the dark ages but yeah.
This is a civilization that had existed for a hundred thousand years.
Another thing too is with all the genetic engineering that you noted Earth aesthetics will be rather ordinary...which ironically might make Tau'Ri more exotic.
Aesthetically any way.
I haven't thought about how much of the census for the Empire is genetically engineered. I need to probably sit down and think about if
Nah in the case of a Super Mac just slap a couple dozen Naquadah reactors on the thing and your good.
As for why we won't see 303s and 304s at least for a fair bit of time its because they're massively more advanced than what is currently available on Earth. I mean sure they'd love to build 304s especially post unending ones but the tech just isn't there and right now Earth needs hull numbers to start building its fleet above all else. Even if the first few ships will be fairly obsolescent as combat vessels in say a decade they'll still have a use as training ships and as vessels to haul stuff to orbit and around the Sol system
There's this too, there's a chance you guys may see the Deadalous design in the field at least before the story ends.