^^ I have done only100 basegame spins and a dozen bonus buys, so comments not based on extensive experience.
Are games like this basically unplayable without the bonus buy?
No I don't think so but base game spinning is usually a bit boring for a game that features bonuses / bonus buys.
The bonus buy gives:
"5 spins + golden symbol
An additional ‘Golden’ mystery symbol is added during the free spins. The ‘Golden’ mystery symbol requires 2 cascades to award one additional spin and transform into a random symbol. "
There is no increasing multiplier or anything like that so I'd argue that the big-win potential in the base game is per se not much worse than in the bonus.
The golden symbol itself only transforms into one random symbol.
Maybe it does something for the timing of the mystery symbols on the real big wins, but I would have to spin a lot more before having some indication for that. Seems unlikely.
Having said that, this game is suitable for 'bonus buy only' playing and for base game playing topping of with a partial bonus buy.
Does the bonus differ if building on Super High volatility mode vs High volatility mode?
No, according to the game help info either way of building leads to the same bonus called 'super free spins'.
This afternoon I started with EUR 200 for minimum stake bonus buys that are EUR 20 each for 200x €0,10.
From a dozen bonus buys I have got some unspectacular results ranging from almost nothing to EUR 25,
until the following final bonus buy that gave me 1 good win on police agents for a return of 18,5x bonus buy.
Doubled my money and moved on.