On France the flat tax on dividends or capital gains is by default. 17,2% of social charges you can’t skip and 12,8% of taxes.
You could opt out to use your marginal tax rate but you need to estimate it.
Also it is good you could avoid the PUMA tax with your partner activity or your part time activity.
My cousin as a similar plan in another ski resort except he will rent weekly or monthly his chalet on AirBnb during the ski season and downsize in a small flat nearby.
It could give you lots of cash-flow by doing a similar approach.
I will not recommend to pay off your mortgage especially if they are long term with a fix low rate.
It will give you some leverage and lower your tax on the rental incomes otherwise you will get hammered.