You got Steel at a good date. At that point the plan should be to just build cannons and other military to support them and conquer as much as makes sense before losing that advantage. China right now is a perfect target because he is back in tech and isolated diplomatically. Frederick cannot plot against you if he is pleased (see guide here:
https://forums.civfanatics.com/threads/civ-illustrated-1-know-your-enemy.478563/). Only Victoria and Qin here can plot if pleased, but Victoria is almost friendly, and even at pleased you can probably enlist her on your side of the war in exchange of a tech. In summary, you had (and still have) a good window to make gains with cannons, but right now your cities are distracted building other things than military. Falling behind in tech is ok and expected during war, but doubling or tripling your land makes it worth it in the long run.
Your current path (Astronomy - Physics) suggests you are going for a diplomatic or space victory, but for space it would be great to have more land (so conquer China first), and even for diplomatic it would be easier if you had more land, to make sure you can win with 2 AIs at friendly for example. Also for space the tech to prioritize is Communism so you can run State Property and use workshops on all your plain grass tiles for mass production.
A few notes about your economy:
- The Great Lighthouse - Colossus combo was a great plan for this map and leader. However Astronomy obsoletes the Colossus and was not clearly needed now, and you are in the Mercantilism civic which directly counters the advantage of Great Lighthouse. You should not be running Mercantilism unless other Civs are running it preventing most of your cities from getting foreign trade routes, or maybe if you have the Representation civic and the science from specialists beats the return from foreign trade routes.
- The Great Lighthouse date was a bit late, aim for it before 1000 BC. Currency (175 AD) and Civil Service (400 AD) were also late, you generally want CS pre- 1 AD. Why was Machinery teched before those? You got Priesthood fairly early but then didn't get Monarchy for some time, is there a particular reason why?
- What did you use your great people for? I see an Academy that is not in the capital or in the best science city.
- This is not a great map for cottages so I assume for the most part your economy revolved around the boosted Colossus water tiles. If so a lot of your non-river land tiles in your coastal cities could be farms (chaining irrigation once you get civil service) so you can run more scientists with Caste System. Alternatively a FIN leader could also build cottages on non-river grass tiles if you do it early enough so they are villages/towns by this stage of the game. But it looks now as if a lot of this land was not developed despite having 11 workers.
- You may be building too many buildings that don't provide much returns. Consider the Bank in Nijmegen. That city generates 30 commerce per turn and your breakeven research slider rate is 70%. So 30% of the time on average your commerce is becoming wealth, and you would get a 50% bonus with the bank. So that's 50% of 30% of 30 commerce, or around 4.5 gold per turn. Since your city could use the 200 hammers from the bank to build wealth instead, it takes 200 / 4.5 = 44 turns for the bank to just repay it's opportunity cost. But it's actually worse than that, because if you were building wealth instead in your cities (when not building units), your breakeven rate on the slider would be higher than 70%, so the wealth buildings (market, bank) would return even less and your research buildings like libraries would return more. Also note that the bank / market bonuses to not apply to the wealth "built" by your cities from production, only the production bonuses like forge applies.
