I think the mis-conception with bonus buys is that its a bonus. its not! Its simply a significantly higher slot spin than you would normally make. If its 100x your money can go up and down a hundred times faster and will be far far far more volatile.
@ChopleyIOM has a youtube video where East Coast v West Coast ate a modest balance in minutes.
You are simply playing a slot game at 70 - 200x ( or greater) bigger stakes, so if you a happy min stakes better like me you bet 20p, and you win various amounts from 0x 0.1x, 0.2x and so on up to the 100x 500x and bigger wins.
A bonus buy reduces those lower hits - but they are still there and at the same time limits the higher wins. You aren't winning 1000x the BONUS buy amount that's for sure.
These are in place to increase spend and play through - THAT IS All. OP says he bought 60 - 70 bonus buys at 70x each. 65 x 70 is 4550 spins at base stake - at 500 spins an hour - thats 9 hours - in bonus buys 2-3 hours. Even if hitting RTP - you are just losing money faster and this is the only reason Bonus Buys exist.
To clarify - with some easy numbers
20p base spin 95% RTP
£20 Bonus buy 96% RTP 100x bonus by
On average you can do 500 spins an hour, or 25 bonus buys
500 x 0.2 - £100 - RTP £95 - £5 hourly profit for provider
25 x £20 - £500 - RTP £480 - £20 hourly profit for casino.
Even the higher RTP is just a con to trap the unwary.