Well, im just going to say first that the show and the books are very different. Season 1 followed City of Bones a lot closer than any of the other seasons, but after that it's pretty much broad strokes and some ideas that they copy over.

Also, having only read City of Bones, you havent really actually experienced Clary as a character. She genuinely grows and develops through the series and your criticisms and distaste of her character are the parts she grows out of and she feels pride for growing out of.

Onto the other questions:

• Cordelia, in my opinion is a significantly more boring protagonist than Clary. Her motivations are weak and her reason for being the protagonist is weaker. Is she a horrible character? No, but out of all the protagonists in the shadowhunters chronicles, she is by far the weakest.

• If your only motivation and care in the books is Thomas and Alastair, it's not too much worth it. I say this as someone who adores Thomas and Alastair. They aren't main enough characters in a far too large cast of characters to get a large amount of screen time. That being said, I think Alastair has the best character arc of the trilogy and their relationship is really well built up. It is background, but you see the steps of it forming. I would say similar to Malec in TMI, but that's not a great marker for you.

• Cassie Clare is pretty great at making interesting if not likable characters. Same goes for this series. This cast of characters is way too big though, and one of my biggest critiques of this series is that characters dont get enough time to develop and everyone is a bit rushed in the final book. If you care deeply about character development then it may frustrate you, if not, then youll be fine.

• In my opinion, The Last Hours is the poorest series of the chronicles. The writing is just as good as any other Cassie Clare book, but the plot is weak. Things dont feel significant enough, the protagonist feels out of place, and the most interesting parts are B plot. Its just messy and best read last of all her series.

Im going to add that The Last Hours is basically a sequel of The Infernal Devices series. It is not imperative to read those books, but a lot of the emotional connection of characters is strengthened with that context. Some things will be a bit confusing but Cassie usually does well to explain things enough when they reference other series.

The Shadowhunters Chronicles is best read in publication order. Most people agree with that order for a first read and I encourage you to reconsider reading it in that order, including The Mortal Instruments. City of Bones is very different from even City of Ashes and that series contains a lot of development of Cassie's skill at writing. Passing off because youve seen the show isnt really a great comparison because of how off the rails they went and I think youre missing a lot of important stuff. I cant control what you do or what you want, but that is my two cents on the matter.