These are just my opinions of course, but I don't actually see 20 hours to convert from 70 hours RAA with XC to PPL(A) as an inherently bad thing.
OP says nearly all that time has been learning to fly the very different 172 after building up 70 hours on a jab. Of course there's bad training anywhere and everywhere and of course it's beyond dumb to let the guy that you pay for lessons decide if you pass the test, but I would have thought you could have this exact experience with a fantastic instructor That could do your RA and GA ticket.
I don't know if that 70 hours is including dual time, but it's still not a massive amount of total experience, and it's all on one type of aircraft. Spending 10-15 hours just getting used to another type is, actually probably a good thing. One of the reasons Cirrus insurance so often mandates that you take the cirrus flight school thing when you buy your plane is because even if you just fly a 172 for 50-60 hours to pass your PPL then do another 50 hours in 172's solo, that SR22 is gonna be a huge leap!
I know it's expensive, but having felt the difference between a jab and a 172, isn't it good to have spend your time learning the differences with a qualified instructor sitting next to you?
For me... I see a lot of people talking about total expense and whether RA -> GA is really cheaper than just GA in getting to your PPL. In terms of the amount of instruction you get for your money, clearly it is. But in terms of total spend it might not be.
What matters is what's the mission - what will you do with your PPL? If you want to just hire/own a 172 or whatever, sure, probably doing all your training in a 152/172 is gonna be cheaper/simpler long term. But if you also want to take out a jab or a foxbat or whatever, I think it's prudent to also do some lessons in that before you do. A jab doth not land like a Cessna, and you might not want to find that out when you're on your own.
Sure the longer you fly, the more planes you fly it will always get easier, I'm just talking about a case where you do all your training in a GA and want to fly an RA or vice versa after relatively few hours.
The only thing that really worries me is a trend I'm seeing in younger schools looking to get competitive pricing.
It goes like this: find a plane that can be RA registered or GA registered. Buy two. Now you use the RA one to get the student their RPC, then convert it to an RPL in the RA registered one. Of course the checkride goes without a hitch because it's basically the same plane. They then use the GA one to get you to PLL.
Much cheaper, maybe $100/hr or more cheaper than using a 172. But you've only flown one very light aircraft. It's on you to use common sense and go and get those 10-20 hours training on something heavier before doing it yourself (though I would hope anyone letting you hire a plane would check what you've flown before giving you the keys lol).
I guess I'm just saying that as long as the instructor is good and you're still learning, lesson time isn't a waste of time or money, and if you only want to fly GA/RA planes, train only in GA/RA planes, but if you want to fly both, of course there's a more substantial switch over than going from a Cessna to a piper.