I also wonder the same. In my layman opinion, the most sensible approach would be moving some of the new nuclear projects to Germany. This would then free up resources in France to invest in renewable power. This could help solve the current problems in Germany (high CO2 output) as well as France (inflexible grid)
This has the following advantages:
* Renewable energy from wind and solar, are the cheapest forms of energy right now, full stop. Decarbonizing our economy is currently constrained by capital expense. So any other form of energy production needs a really good technical reason to get preferred over the cheap forms of energy.
* Corollary from that: Please note that everybody who claims that renewables destabilize the grid by introducing peaks is mostly full of shit. It is absolutely trivial to turn both wind turbines and solar alternators off almost immediately. Why do we try to avoid that? Because having all that bound capital standing around doing nothing lowers ROI (and for example Germany guarantees that the operators will get their lost revenue replaced.) This is literally the only reason. Everyone who claims that renewables at the current stage of the transition will threaten black-outs due to overproduction is spreading misinformation. When we get over ~80% of renewable production, we need new solutions, e.g. to provide fine-grained synchronization for the grid, which is currently done en passant via the rotational inertia of the turbines in thermal plants.
* France has a good position to support renewables very cost-efficiently. Lots of valuable Atlantic coast with wind, both on-shore and with a huge economic zone offshire, a neighbour (Spain/Portugal) with high amounts of wind and solar even in winter months, which is just waiting to be piped into central Europe. But there is no political will because nuclear is so comfortable and you don't have to fight NIMBYs thousands of times for each wind turbine or solar park
* As articles like this one show, France has already hit a constraint with nuclear plants: Cooling water. This constraint is quite scary, as access to surface cooling water will only get worse as climate change progesses. Scaling up nuclear even more would require spreading the load over more shoulders, e.g. by using rivers in Germany. Note that the lignite plants in Germany are cooled using ground water which is collected by pumping ground water to keep the associated open-pit mines free of flooding (with high ecological impact, still maybe a solution for France?)
* Contrary to what nuclear proponents on HN mostly claim, nuclear power is inflexible, and already putting a strain on the French grid. Electricity generation (spot) prices since April are almost always equal or higher in France than in Germany due to lack of transmission capacity between these countries (to verify this, use services like Electricity Maps or Energy Charts which graph the cost of electricity in real time over the last 24h, data is available via ENTSOE-E). Building more renewables would alleviate these problems, but is actually not so easy or economically feasible with the current mostly nuclear grid.
* Contrary to what nuclear opponents on HN mostly claim, base load is still useful to some degree, which is why Germany still burns coal and has biofuel reactors specifically to provide base load. Moving nuclear generation capacities to Germany would reduce carbon output during nights, especially summer nights low on wind, and free up biogas to be used for peaker operation.
Why doesn't France reduce its dependence on nuclear power, allowing economic incentives for cheaper solar and wind power? Mostly politics. Shutting down the nuclear power plants is even more expensive than letting them run until their EOL, which is a tough sell. Also NIMBYs hate wind turbines a lot, and by the decentralized nature of wind, fighting them is harder than the NIMBYs against nuclear plants.
Why doesn't Germany build more nuclear power? Mostly politics and history. Also building a nuclear power plant is not easy from a planning standpoint and might take upwards of 10 years even if society completely commits, alleviating no current problems. Any solution involving nuclear that still makes sense would be a long-term decision, probably pan-European. For now without support e.g. by France, the most economic and climatically sensible path for Germany is to press on 100% with electrification and removing barriers for additional investments into wind and solar.