Hi and welcome here.
This has been extensively discussed in other posts, e.g.
- When is planned 4.4?
- Pls jolla i love you
-
Will you upgrade from Xperia 10 II to 10 III?
…among others.
Short answer → SFOS in Xperia 5II/5III (at least officially supported by Jolla), appears to be very unlikely 
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