I think there is some useful reading here from MSG Legal last year which should be up to date from the legislative changes of the previous years:
It is possible to use Seasonal Leases as an alternative to Holiday Rentals – MSG Legal
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edit:
my reading of this suggests you could potentially have a seasonal let that actual lasts just a week (or even less for example) which at face value doesn't correspond with what most normal people would consider a "seasonal let". Clearly there is an administrative burden on setting such a lease up, but I guess once you're done then you could simply use the same template each time.
I did find a table somewhere about the differences going back a while when I looked. I've found this and it was from here Larrain Nesbitt Abogados but the article is 3 years old and so may not be current. To save reading the whole thing, I've attached a copy of the table to this post - one thing mentioned on that is a requirement to take a 2 month rental deposit for a seasonal let which if the let is only for two weeks seems bizarre. But I guess that is intended as a means to stop people using it as a way around actually obtaining a tourist licence.
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In my mind, I'd still be seeking to get the tourist licence as that is the safest option for compliance and also allows for advertising etc in the usual holiday website channels etc.