I'm surprised that you are saying now that there is no advantage play without bonus after you have wrote
this post where you correctly summarised the ways of advantage play.
I can do the math, really.
I do know whenever there is a player edge or a house edge.
A bugged game in Playtech
My post mainly referred to casinos getting the maths wrong in promotions. With no bonuses, even the tiniest house edge will get you in the long term. I have not been aware that Playtech offered any games with an RTP of slightly over 100%.
Even the best of the Blackjack games has a tiny edge in favour of the casino, even though it is a fraction of one percent. The commonest mathematical advantage play used to take place on such Blackjack games where the WR was set too low such that grinding it out on minimum bets lead to a long term +EV situation such that the expected loss was less than the amount of bonus money gained. This never applied to Playtech since it had phantom bonuses, as do most RTG casinos.
If Playtech have screwed up the maths on one of their games, it is equally vulnerable at ANY Playtech casino. If this is what Betfred have realised, WTF is all this bullshit about asking you about salary and name of employer. They would have shut your account whatever you earned, so they were asking for personal information which was not necessary - in other words, they broke the Data Protection laws. These laws stipulate that a private company can only REQUIRE that customers give them personal information that is necessary for the conduct of their relationship with the customer, they may not "require" more information just because they want it. The provision is there to stop businesses going on "fishing expeditions" for additional data for use other than providing the service they are contracted to supply. Salary and name of employer is personal data, and can ONLY be required if it has a bearing on the service they are providing. Since they had already decided you were "cheating" due to a mathematical vulnerabilty in one or more of the games, they had no need to know your salary and employer details given that the decision had already been made to close your account.
It is clear that Simmo's theory of them just "going through the motions" in order to cover themselves if they get audited was wrong. They were trying to construct a reason to close your account without having to admit you had found a way to consistently beat the base games even without any bonuses.
The proper way to deal with this would be to alter the paytable of the offending game such that the house, not the player, had the edge. A minor adjustment would not materially affect most other players, but it would stop the experienced and disciplined player from slowly getting ahead simply by playing perfect strategy.
The miscalculation has to be pretty striking for you to have turned a string of €20 deposit into a few withdrawals totalling €6K. This does not look like the usual scenario of a game with an RTP marginally above 100% that becomes less than 100% if the player makes even a single mistake.
The Wizard of Odds has a list of all the games from the major softwares, and other than slots, he gives the mathematical RTP as well as the RTP that can be achieved where a game provides for player interaction after making the initial stake, often referred to as "degree of risk".
If there is a mathematically vulnerable game in Playtech, it will be there, along with a strategy card to beat it.
Maybe you can get ahead by a similar amount at other casinos by using the same strategy before they too close your account. THEN you can sell it