Well you can make any house rule you want.
But.... "The time to weave these threads takes the rest of the 8 hour day. The character can not perform any other actions during this time (besides normal eating, sleeping etc). "
In spell all you have to do is "spend half an hour a day inscribing..." and player can do what he want for the rest of the day, so why complicate it more? This ritual is rather simple and not problematic. Its a 5 circle spell so it lasts at least 5 weeks (Spellcasting rank), plus extra threads (2 at 5 circle, +2 weeks per thread), and succcess levels its almost sure that even for a 5 circle magician its gonna be 10 weeks, and if you want you can start weaving another spell before previous ends.
Some spells were created after discovery of spell matrix talents, because you can attune to grimoire only for "...requires a Patterncraft test against the spell’s reattunement difficulty. The attunement lasts for ten minutes per success." I guess this ritual wasnt ment to be cast with grimoire casting. Grimoire casting is an outdated casting method.
Its your choice to houserule it, but the way it is now:
-it takes two weeks (half an hour a day) for the ritual that lets you weave threads
-occupy one matrix until cast
as for Extra Threads I realized that its not the weaving that last for one week, but the ritual that
lets you weave the threads.
So yes, after first week you weave first thread (plus any Extra Threads with high weaving roll), after second week you weave second thread (plus any Extra Threads with high weaving roll), and after those 2 weeks the ritual is over so you can weave all Extra Threads you want in rounds and cast the spell. Simple, easy and player friendly