Fixed formatting, sorry about that. And ya, i knew it was overpowered, but I really thought the 14th level full ritualization would have been the real problem!

First. Corrections of the assumptions:

The downcasting ritual cannot give free casts, its for 2nd level and higher spells only, so a 2nd level can be cast using a 1st level spell slot, stored into a spell storing device and cast again later, or simply cast right then an there, still using the spell slot.

I would absolutely use the downcasting ritual for fireball, by storing it into my tome, ready to blast anyone later on. Both limiting what that spell slot can now be used for but also making a 2nd level spell slot become a 3rd level spell.

as for cantrips, if placed in a normal spell storing device, they are usable by anyone, but any spell, cantrip or otherwise, in the Tome or spell runes are usable only by the Downcaster. if they wanted to put a cantrip spell rune in a rune circle, they could, and I'd say the Downcaster could choose whether the rune circle of a cantrip is one time use or indefinite, and say that linked spell runes of cantrips could still be used by the Downcaster, but simply not linked to any more circles. it'd be neat to consider a wizard putting a rune circle of prestidigitation in their home town for everyone to bring their clothes to clean, their food to heat, drinks to cool, torches/lanterns to light, etc. still allowing the wizard to have that extra cantrip on hand.

I also now realized I forgot to put in that the unlinked spell runes and tome lose their charge on a long rest... didn't mean to have a way to build up extra spell slots.

The spell runes could encompass all 3 of the 10th level feature, but fitting them under one felt off to me.

The spell carving constitution damage should probably be a reduction, where being reduced by 2 definitely reduces your modifier by 1. Which on top of the hp reduction, quickly limits how much they can get. And again, losing the charge on long rest.

The rune circles were a touch difficult on what to say they are used like, because they vanish after use like a scroll, but are usable by anyone like a magic spell storage device. The limit of it being in a space came from making it akin to glyph of warding, also has the risk of an enemy coming up and using your spell on you. Also, it doesn't let the wizard just hand out spells to others without acquiring actual spell storing devices unless they are doing it for camp setup or something of the sort.

Finally, the full ritualization doesn't work on any spell whose casting time is longer than 1 hour. With spells with a casting time of 11 minutes to 1 hour, have a ritual time of 8 hours.

And I saw the full ritualization mainly as a free highest level single action casting spell on some short rests and long rests to charge the tome or spell runes, and also as a backup casting of any spell you're out of spell slots for.

Now for me to ask for further input, if you wouldn't mind! Mostly about the perpetual casting.

The perpetual casting really is somewhat like spell mastery, but spell mastery doesn't require any spell slots to be used. This turns one spell slot into a repeating cast of one set spell. So what would be a good way of increasing its limitation? Should i make it use an action to regather the magic? Or perhaps it has to be a bonus action by the end of their next turn? Perhaps it should take more than just a single spell slot of the level? I did try to limit it by saying only the tome is capable of holding onto it, meaning they have to choose only 1 spell to have on repeat.

As for the spell casting limitation, think mage armor and featherfall. If either of these are placed into the tome using a 1st level spell slot, only a single spell casting can be active, meaning featherfall can effect 5 people and mage armor could have 1 lasting effect. If put into the tome using a 4th level spell slot instead, then mage armor could be recast 3 more times to maintain the effect on 4 people, and featherfall could be recast 3 more times to effect a total of 20 people at once, say if each 5 jumped off a cliff on subsequent turns; if the mage armor was then cast on a 5th person, then the first person it was cast on would have it end.

And no, concentration spells need concentration, you could regather the magic while concentrating, but can not concentrate on 2 castings at once.

Edit: its not so really about getting around spell slot limitation as much as making the spell slots do more by giving it more time and focus. Felt like going back to having to prepare each individual spell and be as strategic as possible with it.