If it is just a plan then a vertical rise at the end certainly
does not comply with the Saniflo instructions and thus would
undoubtedly invalidate the warranty.

Indeed. The Sanisplit, on the other hand doesn't have this restriction,
which is why I would recommend it if this pipe routing was chosen.

Of course with a Saniflo it might well be easier and better to go up
from the first floor cupboard and through the loft (assuming a two
storey house) to the stack. This would avoid a lot of difficult work
under the floor and could all be done in 22mm pipe, assuming the
length is suitable.

Yes, that would probably be a preferable routing if feasible.

Christian.