As usual, I'm unfashionably late to the party Let me start with how cool the foundation of your chapter looks. I'm a sucker for a knights in shinning armor story, and yours fits a great niche in the 40Kverse. It's not Imperial Fists/Black Templars Knightly or Dark Angels Knightly. It fits right in with the classical literature (Ivanhoe, Le Morte d'Arhur, Song of Rolan) and classic movies (El Cid, Excalibur, Kingdom of Heaven (director's cut)). I'll just comment in post order.
Chapter Culture, Ethos, Combat Doctrine all look good. I like the formality of the culture, which fits in nicely with the Medieval feel. Your "Never Temper, Never Relent, Never Forsake" ethos is also very fitting. This will make for a great test of character and ingenuity when the Chapter is in a tight spot. Combat Doctrine looks solid. Range over rage looks sound. Lack of rage is not a problem. The rage is more fitting for the likes of the Space Wolves and Blood Angels foundings as it is so much a part of their legacy and genetic make up. I do think there's room for individual and squad level knightly quests, and I hope you will touch on that more.
Yep, I'd like to have quests involved in the Chapter lore/beliefs. Some penitent quests for marines/squads/etc who've screwed up for example.
It's all very fitting for rank, ceremonial, and honor titles, so I hope you keep those. Titles like king, baron, earl, etc. might be pushing the obvious a bit. I think you can come up with something a little more exotic, yet fitting to the theme.
I'm thinking of each company have an "order" title as well though it might not always be called such - eg the 1st Host are the Knights of the True-Iron banner (and there'll be some background and rationale for each). With them will come titles that should be suitably archaic and fancy. I think a few of them will be earl/king/etc but "spruced up" and not consistent across the Chapter. These weren't designed after all but developed and earned over time.
Chapter history is good. However, there are some areas that need tinkering with:
The Shield Argent Chapter’s surviving records date back to the 35th Millennium however the Chapter is known to have been born in the Second Founding amongst the numerous scions of the Ultramarines.
Second founding chapters are pretty well established. This isn't necessarily a problem if you expand the explanation beyond lost records. Perhaps third or forth founding might be easier and more credible if you want to keep the founding story tight and simple.
Khera’Nex xenos type
I may have missed something here. Are these of your own invention or are they established in 40K lore somewhere? I'll be very embarrassed if it's the later If it's the former, you should expand on it.
Barrow Tell, a vast and ancient piece of machinery dating from the Great Crusade
This sounds more like dark age tech than crusade era tech. Perhaps the Khera'Nex were fist encountered earlier in human expansion and thought expunged as a threat long ago.
It all looks very promising and I look forward to seeing more
Yeah, not sure about which founding they would have thought they came from. Has to be very early but I guess it doesn't need to be 2nd per se. Maybe I'll just lump for 3rd and be done; it's still so far back that the history could be lost. Being as they are actually 2nd founding from the DA that'll make the sudden change to everything they thought even more wrenching.
The Khera'Nex are mine rather than canon. I need to flesh them out more and put a side box/info thingy in my final IA. Got a bunch of ideas for them but not nailed it down yet, maybe a good topic for a future post; get the crowd's view and input.
The Barrow Tell fortress is a key part of the lost history of the Chapter (which I shall expand upon shortly - work allowing) and the two mechanicus machines inside, the emitter and the predictor, both date to at least the Great Crusade and were originally ship-born. I couldn't decide whether they should be pre-crusade tech or the result of a particularly ingenious tech-priest who designed and operated them. These days the functions of the machines would be mixed up amidst far too much ritual and they are therefore barely used correctly let alone to their full potential.