So, depending on the focus of your coven (battlefield/being nosy/quests/info gathering), you would want everyone to take some stuff they can do solo, and then depending on coven size a few rituals that you can just about manage if your whole coven gets together (showpieces basically). You will probably want most of the bases covered below 10 magnitude by at least one person. Bright Lantern, Horizon’s Razor Edge, Piercing Light & Cyrstal Clarity of the Rational Soul. If you are going for Day being a strong part of your group’s aesthetic, then someone to have Solace of Chimes and cast it on your group tent if you have one at the start of each event.
Above that you are getting into Coven rituals - Scrying is useful to get yourself known and get your foot in the doors as a coven, especially if you can cozy up to your nation’s generals to scry where they want in a timely manner. Economic rituals are also useful if you can get clients willing to pay - try and get mastered cost + at least half a crown. If you can do multiple then great, pocket the free crystal. The mines one used to be better because it was a bit broken but they fixed it. Sular’s Promise has competition from the Autumn one that is cheaper so that’s unfortunate. If you can get some regular clients then great, especially if they are visiting ports that have a relative jump at +6 ranks (ports are slightly uneven in their rewards distribution).
If you’re going to be leaping into every mission & plot you can, then investigation stuff is very useful, but you’ll have to be proactive to make use of it, ideally being active within your nation’s plot will get you chances to go and do things. Skein of Years, Bright Lantern are more useful than Clear Lens in most situations.
If you’re in Dawn and willing to take the battlefield, get 3-4 people with Chimes of Annulment.
All the World In a Grain of Sand is awesome and the last word in a curse fight, but its also one of the most expensive curses out there and basically just useful in PvP.
If your Coven is even close to big enough to cast Clarity of the Master Strategist than master it and make casting it your main goal. Once you can do it, do it as often as you can. If you are that size also consider everyone mastering Piercing Light to destroy enemy Shrouds.
Items-wise, you are mostly looking for lore boosters like rings and staves. If your nation is herb rich, maybe look for some personal-mana-as-crystal-mana potions but they aren’t cost-effective unless herbs are cheap and plentiful. Other than that, covenstones can be situationally useful, but only worth the investment if you’re making regular use of them. Then general gear for fighting, wands of gain spell, etc. Impale for paralyse rod if you’re a bit cheeky and like giving people nasty surprises.
Pick a magical tradition that best suits your Nation is always good advice. Day works pretty well with Astronomancy and Dramaturgy IMO, although Runes are always a solid bet. The stuff which shows your breadth of specific knowledge looks impressive with Day, and the above traditions are all symbolism based and have a lot going on with them so if you can commit some key bits to memory that’s great.
In terms of stuff to do - In my experience, Day Magic practitioners benefit from having a lot of information from varied sources, to cross-reference and link together to solve problems. Various Day rituals can extend that ability, but having your fingers in lots of pies and gathering and sharing information will massively extend your reach, and your ability to ask the right question at the right time (when doing Swim Leviathan’s Depths for example). So have some people involved in your nations politics, the Synod, Conclave, the Military Council, or just hanging around the pub. Then periodically meet and share what you know with your coven, so that people can start making links and pulling the coven as a whole into all sorts of activities. Let the Archmage know your coven exists, and if you’re up for scrying, the Warmage and Imperial Seer - THEN FOLLOW UP AND STAY INVOLVED. If someone from you coven comes up to them once or twice ever, they will forget about you almost immediately, so you need to keep engaged and be helpful, since plot opportunities will blossom around them.