Good going!
First thing I'd like to point out has to do with the early game trade offs.
On turn 75, you're only seemingly behind in techs.
We have researched about as many beakers as each other. If I have a headstart on Currency, it is solely because I got better value off the Alpha trade (backfilling).
Besides, you can actually research faster than I can, for several reasons:
- I took more and earlier maintenance hits than you did (settling cities), so I bleed more gold than you do;
- My Empire is more widespread than yours, which means I lost many worker turns moving my workers over to new settlements;
- You got a healthy early workforce combined with Pottery, which has allowed you to start cottaging.
On my end, I have more resource specials, especially gold tiles.
But your 6 workers + compact and already well developped Empire means you'll be able to cottage faster than I could.
So, imho, for the turns to come, your commerce output should rise faster than mine.
Of course, a larger Empire has better long-term potential... but this can be made up for with a timely invasion.
If you can invade a neighbour before reaching a plateau (you should be able to), then your long term potential will rise again.
Those are dynamics at work.
It's rare that there's a single best line of play. However, there are means to optimize a line of play.
So, to finish with this point,
I took your save and quick played to Currency. I reached it 2 turns later than with my save. But I was then doing about 30
/turn more with your save. Not even considering my extra maintenance.
With proper management, you might be able to reach Civil Service before I could.
So don't be jealous and assess the strong points of your game: the superior workforce to develop the land.
With this said, we can look at some shortcomings.
First, I'd like to draw your attention (again) to the number of roads you've built.
Some of those roads are useful, some others aren't very much.
I count 18 roaded tiles, which equals to 36 worker turns.
36 worker turns can be converted into any combination of 9 chopped forests and grassland cottages.
If you look at your map and add, say, 5 cottages and remove 4 forests for chops, maybe you can get an idea of the cost of roading so much.
To work those cottages, you'd need more population points but it's conceivable you could have them. Chopping forests saves time on stagnation (while building workers/settlers) and allows a faster growth.
Which leads me to another point of your game.
You've been stagnating Hamburg at size 3, working corn and hiring 2 scientists.
I can understand how appealing the faster Academy (or bulb) can be, but I think this is a case of mismanagement.
Hamburg has a Granary, a happy cap at 6 and many unworked food positive tiles (!).
Say we assign Hamburg corn, farmed floodplains, cottaged floodplains.
The city can then grow 3 sizes, up to size 6, in a mere 5 turns (!).
As it happens, you also have a considerable workforce in position to cottage the land.
So, at size 6, Hamburg could work Corn, cottaged floodplains, cottaged grassland x2 and 2x scientists (freeing up the farmed floodplains for Frankfurt).
Going such a route seems much stronger to me.
Sure, it delays your Great Scientist, but it makes the most of your already improved tiles, infrastructure and happy cap.
So, for the following turns of your game,
I think you should focus on your workforce (it seems to me it's a bit out of position, as if you got disconcerted by how many workers you had).
What I'd be most attentive to are meaningful improvements. I think you can go full on cottages and chops (mostly cottages).
It's important to improve tiles you can work in the short term. And when you do, it's important to focus on growth.
Re: chops: there's no need to pre-chop if you're not going to need the hammers sometime soon. Are you chopping something in Hamburg?
Another key objective for your workers, for the turns to come, is to connect your trade network to a neighbour. You can make good use of foreign trade routes (those are roads that pay for themselves).
Arguably, this should already be done.
Otherwise, you mentioned the barb city. You can aim to capture it.
I think there's room for two more cities (maybe more?), so you may look up for a place to start another settler (Munich? Hamburg?).
Cologne has very little food surplus, maybe that riverside grassland is better off farmed. It seems to me that having the pop to work the gems is more important than the extra cottage.
Once you get Alphabet, I'd rather set research to Currency than Construction. Your economy could slow down a lot if you went for Construction and it isn't like you need War Elephants to capture cities (Swords and Macemen are options).
You can build Research if you want to to speed up Currency. It may be better than Chariots but not necessarily better than Axemen (to capture the barb city).
You killed a fish tile when you settled Essen. Slight misplay, there. Settling 1N would have been better.
ps :
The settler coming out of Hamburg then settled the Frankfurt site, which could immediately make use of the now-orphaned tile improvements around Hamburg like this: grow to size 2 (2/24 ) working a floodplains farm while building a granary, then grow to 12/24 on the farm and a floodplains cottage, swap to copper/grass mine for a turn to push the single-chopped granary over the 30/60 threshold, and whip with the food box half full to regrow almost immediately on the pigs.
This is a pretty good play. Combining a 1pop whip at size 2 with a chop to get a fast granary is very efficient.
Especially when there's a strong food special at hand (granary = double food output).
If you want to do it perfect, however, you should aim for a food bin of 11/22 after the whip. (more or less, slight difference, no need to forego working strong tiles to get exactly there.)
So, maybe you should have started working the copper a turn earlier.
Whipping with 5-8 food in the bin and doing +5F for the turn would be very good.
, since I'm even less eager to convert 
into 
I think I have overvalued this site, though -- I've noticed the gold city on the eastern coast but didn't consider it quite as worthwhile; it pays for itself by working the gold mine, but it takes 25 turns to return the investment in 
. With all my cities low on food, however, that may be as good as it gets.
-- but I'll already say that I find your style impressive; the AH -> Sailing tech path combined with your dedicated push towards all those coastal/riverside gold cities is precisely the kind of skillful, flexible play that I'm hoping to see and learn from. I tend to neglect my horizontal growth, so I need to see how much more can be possible there.
/
Unfortunately, if I were to research Masonry next, I would be delaying the library further; if I delayed Masonry, at least three Industrial leaders on the map could spoil my attempt, although stone-accelerated failure-gold would unquestionably help my economy as well. In addition, chopping so many forests shortly before a Maths bulb seems rather incoherent.
-- with slightly questionable success. 
