Then I read your answer that it is better to park the city in the Modern era.
This is not at all what I said.
Q: So is parking in the Modern Era better than parking in the Industrial Age or the Colonial Age?
A: This depends on what our current, and most important, gaming objective is.
If we already own all of the GBs that I recommended to construct and level up between HMA and Colonial, and we already have them all at level 10, then yes we should be moving up to higher eras. We should park in each era along the way, at least long enough to score diamonds from each recurring quest. In this scenario, we could park in ME a much longer time while producing a mountain of ME goods that we could either trade up for later-era goods (reducing the time we need to park in each era after ME but before FE), or we could trade ME goods down for more PE goods while selling both ME and PE goods in exchange for FPs donation to our GBs. All the while, we would be also scoring mad amounts of medals with which we could expand our city faster. For the purposes of producing Goods and/or Medals, heavy questing is just as effective in the later eras (provided we have leveled up our GBs enough to afford spamming UBQs) as it is in lower ages. While we score fewer packages of medals and goods, each individual package is worth more total value. For example, I might only score 1/3rd of the total goods packages here in FE as compared to how many goods packages I used to score per-day down in CA. But with my level 10 CF bonus, I'm scoring 13 FE goods per quest reward, which is so much more total value than scoring 13 CA goods per quest reward is. Therefore, it's not as much of a drop-off in total value that I produce fewer goods per day in FE, since the higher eras goods I am producing are that much more valuable.
However, if we have a city full of under-leveled GBs (below level 10), then what we need more than anything is more FPs that we can to swap to our GBs so they can level up faster. The Colonial Age is the best overall heavy questing age in this regard, since it allows us to complete the most total quests per-day of any time period in the game. The more quests we complete per-day, the more times per-day we score packages of: goods, medals, or forge points. This means we will score more 5 packs of FPs in CA than in any other time period in the game; the same goes for goods packages and the same thing goes for medals packages. As we advance to each period later than CA, we start to lose the total number of quests we can complete per-day, which means we will score far fewer FP packs per-day, which means we will have fewer FPs from quest rewards to swap to our GBs, and they will not level up as fast in the later eras. So my advice is always remain in the Colonial Age until all of our GBs are level 10, because that is the age where we can produce the most FPs per-day, with which we level up our GBs through FP Swaps.
So if 100% of all your GBs are not level 10, then do yourself a huge favor and park in the Colonial Age until they are all level 10, it will help you so much more when you advance to the later eras. This is especially true for any GBs we will construct that pay a 24-hour FP production such as: HS, CdM, IT, and AO. (I'm not a fan of constructing Cape Canaveral at all and I will post why in great detail at a later date.) By pushing these GBs up to level 10 in the lower ages, when we have access to more FPs-per-day through quest rewards, we are also helping our later-era advancement by increasing how many FPs-per-day and FPs-per-tile that our city can produce in those higher eras. So in the later eras, when heavy questing will produce fewer additional FPs per-day for us, our FP-producing GBs are ready to step in and fill that gap in lost daily FP production.
If you have already advanced past the Colonial age, and you have a bunch of under-leveled GBs, then stay parked in the Industrial Age until your GBs are level 10. The "Produce Supplies" quests in Ind are "Produce 4 x <24-hour productions>" and there is a still tiny a 2x3 Gunsmith supply building to quest with (see my Ind city plan I posted earlier).
If you have already advanced past the Industrial Age, and you have a bunch of under-leveled GBs, then stay parked in the Modern Era until your GBs are level 10. The "Produce Supplies" quests in ME are "Produce 5 x <24-hour productions>" and there is a modest sized 3x3 Hatter supply building to quest with that only requires 1-lane roads.