Glad you enjoyed, OP! I absolutely agree with all your positives/negatives. I have fond memories of my time with Syndicate, it's one of the few AC games I've managed to finish. Jacob and Evie were amazing, and Jacob was a pleasant surprise considering how irritating I was expecting him to be. (Stupid Sexy Jacob.) Henry and Abberline were really fun, too. London was beautiful and lively, and is still one of my most memorable open world experiences. There was such a distinct separation from all the areas in the city, but it felt very subtle at the time when you're walking through it. It's funny, my experience with AC:S is a lot like Skyrim where I've blanked over a lot of the story, but I can remember all these wonderful little moments from exploring the world. The markets, the backstreets, the parks, jumping across the Thames... the boys hanging out on the roof with the cat that I may have accidentally pushed off... Syndicate did have some memorable missions, though. I just wish the main story line had a lot more meat to it, and the game had a lot less collectables.
It's a shame the main story was so lean, especially when the villains was so well done in terms of acting and mo-cap. Starrick and Roth were the absolute standouts. The dynamic between Jacob and Roth was surprising too, and it's tiny small baby steps for Ubisoft to have both female lead and a bisexual male lead, though the latter is only really hinted at in game. (In retrospect, Jacob did pretty much flirt his way through every scene with Abberline.)
The Jack the Ripper DLC was almost more memorable than the main campaign, so definitely give it a go, OP! I keep hoping we'll see more from Jacob and Evie someday, but that's probably not going to happen. This thread made me realise how much I missed playing Jacob and Evie, and rope launching my way around London.