One aspect of seidr, is that Odinn received knowledge of the runes the knowledge by sacrificing his eye and hanging by his foot on Yggrasil for days.
So while in SSSS the way they teach it is the school - there is reason to believe a mage can receive the wisdom in other ways.
Reynir already showed signs of this in adv 1, it may have been a direction for adv 3.
I really wish we could have seen an experienced seidr mage. Either Reynir properly trained or maybe meeting one in Norway.
One other thing: magic has a cost of some sort. Minna did actually confirm that this holds for Icelandic as well as Finnish magic. We don't know how this works, but it would imply a limit on how many functioning runes an Iceladic mage can produce. Perhaps a more permanent rune might have a higher cost? Or perhaps a mage can be responsible for only so many active runes at once, and has to choose which ones to maintain?
Just dumb ideas, here.
Also, as Roisin said, we do know from books on the runes that in many cases just having a drawn rune doesn't produce the desired effect: one also has to use a spell, which can have a limited duration. I am not sure whether a rune can be reused for many spells, but I find it amusing to imagine that Iceland has 'public runes' that can be used by any mage to cast their spells. Even runes of 'finding your way' carved on rocks in places where people get lost a lot, that sort of thing.
I think that the magic is heavily based on actual folklore - so from my knowledge of Norse runes it would definitely make sense for there to be public runes and things like that. There's a few runes that are found over and over again on stones in Scandinavia.
For the cost - one thing to consider is that ultimately magic comes from the gods, so I think that how much the gods support you/your intention can impact how much cost the human has to pay.
So Reynir I think got a *ton* of support on magic use in adventure 1 because the gods really wanted those ghosts released. He was able to create loads of runes without any seeming difficulty or fatigue - whereas Lalli, a far more experienced mage, ended up incapacitated twice.
Lalli has no connection to the Norse gods who rule Denmark and I don't think he got as much support as he would have in Finland.
Then Reynir wasn't really as useful in the second adventure. He only, iirc, had one functional rune and it wasn't terribly strong (the "walk around us" for aimless wanderers)
Then the swan definitely supported Lalli and Onni in the second adventure. At the end literally guiding Lalli and Reynir step by step, supporting them throughout the Kade - I imagine that a random mage with no divine support would have a way harder time navigating a Kade.