Short answer: anything above 20% is unproven and will cost significantly more if you expect to have 100% uptime/reliability. If you're okay with 95% uptime, wind/solar can get the job done, and I believe cheaper based on
https://www.sciencedirect.com/science/article/abs/pii/S25424...____
The flaw with renewables is they are so dam cheap and so dam finicky. Basically, renewables force nukes and nat gas to sell power at a loss or sit idly by for most of the time. This becomes worse and worse as the renewable's market share increases and they eat away at the profitability of the dispatchable sources. The limit is something like 20% at which point nukes and nat gas just can't make any money and so a free market system would just get rid of them. But yet, they need to be there, just in case, because they are dispatchable and you want shit to work at all times. Unfortunately, prices for consumers don't go down because you need the super dispatchable power sources which just sit there for a lot of the time and cost just about the same if you have em on or off, and you also just built a bunch of renewables which are cheap and make money for the guys who built them, but hurt the rest of the grid because you have a bunch of idle equipment (gas/nukes during the day and most of the year, panels at night). Lower cost storage is a way to address that, but has to be really low cost, especially when you increase wind/solar market share. Keep in mind, there's not yet a good solution for long term storage (days or weeks or months).
To go beyond 20% renewables you need a power market that rewards dispatchable sources, essentially preventing solar and wind from taking all the money and bidding everyone into oblivion. If the market is fair, there will be little incentive to push renewables because they just make things more expensive. There's a bunch of ideas on how to do this like capacity payments. Might work, but it's gonna cost more because of all the unused equipment. Everything is possible, you just need to be willing to pay for it.
In an 80% renewables grid, most of the time, renewables will be meeting 100% of demand and producing much more than needed. 20% of the power is delivered by nat gas. But they will mostly be called to do that only on the bad days where they will have to be able to match closer to 100% of the demand. So you've basically got two nearly complete fleets. Who's gonna pay for the idle equipment? The solar power producers should right?
One data point: Germany is 30% renewables and pays 50% higher than EU average. That's old prices in a country not ideal for renewables. It's no Arizona or Washington.