In the late game, a town offers 6

for financial leaders, or 8 with free speech.If you are going for space your good cities will want libraries and observatories, with only a few building universities. Labs do not count as they come too late to pay back on research, they are only built for the production modifier. This results in a 50% modifier for science, or a town being 9

Your capital (oxford) will want to stay with towns in any case.
OTOH production will have a 110% modifier, due to 10% from state property. A workshop with caste makes 4

, resulting in 8.4

. This means that for non-financial leaders workshops are strictly better than towns (edit: while running SP+Catse and neither of US/free speech), while they are worse for financial ones looking at raw yields, production is more flexible. For instance, you can put the same amount into wealth, meaning gold, which you can not with commerce, due to missing buildings.
Now let me explain on the civic choices. This starts with the observation that cities will and should have overlap. On average a city can maybe work 15 tiles. This means that cities do not need to exceed 15 population on average to work all good tiles. In turn that means that if you have managed your diplomacy you will not get into happiness trouble from a typical +6 unhappiness from emancipation, meaning it adds very little, while preventing being in the great caste system. Thus I stay in caste, unless forced by the UN.
Then there is the question of bureau vs. free speech. Free speech will make add 3

from towns, which is significant. OTOH bureau adds +50% commerce in the capital or 3

from towns + additional commerce from other sources. At that point you want your capital to have oxford, meaning it is much better than other cities at converting

to

, and thus will greatly benefit from bureau. What it comes down to is how many matured towns you have. The more you have the better free speech becomes. There will be situations when running free speech and keeping towns will be the better move, but for that you need a lot of grown cottages. For a space-race, keeping the towns has the disadvantage of lowering production, which you will need for ship parts, meaning that they will have to be replaced at some point, but by that point you typically can build a huge worker force in a few turns, so that comes relatively late.
towns vs. workshops is sufficiently covered above. It basically is the same decision as free speech vs. bureau.
US adds 1

to towns, which is nice if your are planning to keep them, but again, long term you will want to switch to shops for higher production. The problem for US is that it needs democracy (4732

) or the mids. The next best choice is rep, which helps keeping the most important cities happy, and adds a bit to research, due to the few specialists you will be running.
SP vs. free speech basically boils down to wanting to go with corps (especially mining inc.) or not. I greatly prefer the +10%

and reduced city maintenance from SP to the additional trade route from free market.
The problem with mining inc is that it needs and engineer, which is hard to get for certain. It is also a significant investment, and needs a lot of resources to beat SP+shops on peak production. I basically never use it.
A few more thoughts:
You'll need Communism before Plastics, which can slow your Plastics date a turn or two. (Unless you're able to do another trade.)
IMHO you greatly overvalue the TGD. Two

per city is nice, but really not that good, as there will generally be some available food and bad tiles, meaning your cities really do not have to be that big to work the good tiles. Also you seem to want to build it in your best production city (?), which is a big mistake, as that city should be kept clear to build the internet a few turns later. The internet really is the only big late-game thing for a space race, besides ship components. Also the AI loves to go after TGD, meaning you need a substantial advantage if you want to get it. They are generally less eager to go for the internet. TGD also comes at the cost of 1750

, which is about 1/2 to 2/3 of a turn.
All in all it is a nice wonder, but far, far worse than the internet.
Does state property get you the two wonders faster?
It does, due to significantly increased production.
It just seems like a lot of work to figure out where to build workshops in the right places and the benefits don't seem overwhelming vs. spamming farms, mines, and cottages when you're financial.
Especially if you are financial, but also in the general case, I prefer windmills to mines, due to 1

and 2(3)

beating 2

. The food is the big thing here. Also when workshopping, the oxford city keeps its towns and all other flat land gets replaced with shops, and watermills along rivers, due to them being 1

, 3(4)

-2

wrt. shops. Both mills can be replaced when building ship parts.
You might lose a bit of research from giving up e.g. Free Market, although you'll pay less maintenance.
Production is science. This late in the game I usually am running a few cities with wealth to get to 100% science and let the rest build science.
Resources (say +15 health if you have 8 out of 11 food resources, and all 7 resources for granary and grocer), fresh water (+2 health), aqueduct (+2 health), base health (+2 health) = +21 health.
+ 2 from public transportation.
You probably don't want to replace towns, because tech rate matters and your production rate is already decent.
As I mentioned several times, at some point you will want to, because the tech rate stops to matter and pure production starts.