It should be fairly easy to build a net zero offgrid energy house in the south of France as winters there are fairly mild. Is that normal that a build permit there take so long to get ? It took me just about a week to get the building permit here after I applied.
There will be very few reasons to use Victron MPPT's other than to get back that 20% efficiency for using 72 cell cells instead of 60 for battery charging but likely most of your energy will go directly to heating and thermal storage and not to the Lithium battery.
2000 liters of water used in the +25C to +80C range 55C delta can store 2000 * 55 * 1.16 = 127.6kWh but the house depending on how it is build (if there is reinforced concrete and bricks) withoutside thermal insulation then they may be able to store even more than those 2000 liters of water.
Winter is when you will need most of the energy and not sure how good of an idea is to have the panels on the roof depending on roof orientation and type.
Say if I take Lyon as that will be the worse case for south of France
Looking at PV watts the solar production in winter is terrible so you will not benefit much from thermal storage and what you will want is to have a large enough PV array and only overnight thermal storage (that can easily be just the building envelope depending on type of materials used). Apparently a 10kW PV array there will produce just 250 to 330kWh/month in the 3 winter months and average temperature for January is +3.2C
The 250 to 330kWh if for roof with south orientation and just 20 degree tilt. Having panels flat mounted on a vertical south wall will provide you about the same amount in those winter months.
But if say is somewhere around Montpellier then you get double around 600kWh in the winter months from same 10kW PV array either roof mounted or flat on the south facing wall.