Jay - I would say the main issue with Gwen Moore's book is that is is written for USA consumption and a lot of the time plants that grow well in the 'continental' climate there don't do so well in our cool, temperate maritime climate. Many plants need a hot bake in summer to develop internal changes to their body chemistry that allows them to overwinter at colder temperatures. There is lamentably nothing sensible and UK-centric in print at the moment.
Dudleyas. I currently grow cymosa, lanceolata, caespitosa which I believe are about the toughest, also edulis, green and white forms of brittonii, and an u/k hybrid. For several years I grew pulverulenta but that died out one summer. I actually think any are worth trying.
Aeonium spathulatum is the hardiest of the genus. It has even self-sown here. A. simsii is also pretty hardy and there is a hybrid between the two, x barbatum, that is hardier than either - I have been growing that outside for over 20 years. A. simsii is used in hybridising a lot as it usually gives those nice frilly leaf edges to its offspring and they often inherit the hardiness, too. There are a couple of hybrids between simsii and 'Zwartkop' that are great. In habitat simsii is only found at high altitude, generally, on the north side of the rocks it grows on and, at times, the weather is wet, misty and freezing. A. smithii is also very hardy but tricky outside. I grow a few other hybrids. 'Velour' is settled into the trunk of a palm and growing nicely with a convenient canopy of leaves to keep the radiation frosts at bay. What I tend to do with aeoniums is snap a head off and poke it into a rock crevice and let nature take it's course. I have several that have gone through the past couple of mild winters and are now nicely established. A proper frost would see them off, of course, but I still have the mother plant if it is one I want to continue growing.
These are all winter growers, btw. In summer dudleya kind of shrink, especially those first ones on the list. Aeonium spathulatum, simsii and smithii virtually disappear to tight little buds, Aeonium hybrids tend to not grow but colour up nicely with higher light levels. Come cooler temps and wetter conditions they all start growing again.
Echeveria secunda glauca is indeed one of the hardiest. E. elegans particularly so - I started with a single plant 30 years ago and it now forms quite extensive patches of ground cover. I'm also growing it epiphytically in a yew tree. E. rosea, too, is extremely hardy and colourful. I grow several others, including lilacina, prolifera, purpusorum, 'Perle Von Nurnberg', x imbricata, agavoides - although the latter does best with a cover for winter. E. multicaulis has been fine for the past two years with the caveat that winters have been mild. A lot of these hardier echeveria are from areas of cloud forest in habitat and actually enjoy cooler wetter growing conditions as long as they have drainage.
If winters continue to be mild a handful of smaller aloes are worth trying. You have brevifolia, worth trying aristata, x nobilis (you'll most likely see this as mitriformis), humilis. The grass aloes ecklonis, boylei, cooperi are worth a try. And polyphylla, of course, although you can't call that small. Not forgetting the hardiest of all is striatula but that gets big.
Agaves. A. americana is a huge monster. There are many, many nicer and more appropriate species to try IMO although they are mainly larger than you could accommodate, I suspect. If your filifera is indeed filifera, not schidigera, then it doesn't get too massive.
Lampranthus spectabilis will be fine down to around -5 or -6C but not much lower. Cuttings taken as insurance are easy to strike and quick to flower the next year. It is actually best trimmed after flowering, too, to keep it compact. There are several delosperma and drosanthemum that are hardier but not as showy.
For shadier moister areas you could try Umbilicus rupestris (a native) and U. oppositifolius. They both spread but not alarmingly.
Growing in pure grit is absolutely the way forward.
For me, once the bug bit, I started to travel to see plants in habitat and I now mix it up and grow terrestrial bromeliads and xeric ferns (Cheilanthes, Astrolepis, Pellaea as well as native ceterach and trichomanes) in with it all - trying to get communities of plants growing in a naturalistic way. Increasingly bulbs, too.
When the pandemic has (hopefully) passed I would invite you to visit to my garden - I think you would enjoy it. I've spent the past 30 years doing what you are doing now - finding out which species do well for me. I can certainly pass on lots of cuttings of stuff for you to try and chat about what I have tried and lost, which would take too long to write about.
Finally - I wrote a series of six articles for the Cactus and Succulent Review, starting in March 2015, which you might find of interest.
https://www.cactusandsucculentreview.o ... -2015.html