Could I say paljon instead of tosi in this case?
No, that sounds extremely odd. "Paljon" would need something more concrete, a noun, to support it. "Hauska" is an adjective.
The only suggestion it gives on mouse-over is paljon and then it's wrong?
Why paljon doesn't work instead of tosi?
Reported that a tip is missing and that the paljon tip is misleading on 4 September 2020.
Is 'todella' possible instead of 'tosi'?
Yep! Todella is more kirjakieli (writen language) and tosi is more puhekieli (spoken language)
what is wrong with "meillä on paljon hauskaa japanissa"?
What about using "Kova" instead of tosi?
No, that means "hard".
I suppose it's like the (UK) English phrase 'such fun'! ('We're having such fun in Japan' -- rather posh or middle-class in tone, however!)