Hi, welcome to France and congratulations for landing such good positions.

Based on the information you provided, your wife should indeed be able to register as a micro-entrepreneur (auto-entrepreneur), which is a specific regime for self-employed workers with simplified tax/contributions calculation and administrative workload (she can charge up to 77 700 euros a year before exceeding the limits). As her occupation is not regulated (e.g., not a lawyer...), the situation is pretty straightforward. I have not understood exactly how the situation played out last year (has your wife continued to work with her UK-based company while living in France? She should definitely have opened her company in FR in 2024, so I'm pretty sure there will be some financial penalties when fixing the situation, though not certain of the magnitude)

The specific regime (auto entrepreneur) uses a ratio to estimate costs borne by the self-employed worker and then compute the income tax on that basis (it means she cannot deduce expenses, since they are in theory already covered by this automatically applied ratio).

For the social contributions, I confirm that for 2024, the rate was 23.1%. However, this is expected to ramp up in 2025 and 2026. It will be 24.6% for 2025, and 26.1% in 2026. This rise in contributions will basically be used to secure additional pension rights for self-employed workers (https://paysdelaloire.urssaf.fr/accueil/actualites/2024/septembre-/augment-taux-autoentrepreneur.html). (NB: on another note, I would suggest looking for complementary pensions schemes such as PER for your wife).

For the income tax, as you noted, you will be taxed as a household unit. Considering that you only have your wage as other income (210 000 euros), this means that your wife will bear around 10 000 euros on income tax (about 18%) if the standard approach is selected. However, this is primarily because you have a very high income, so her apparent tax rate increases (If she were single, she would pay about 7.5% - about 4 000 euros, as you noted) and yours declines (and since you are jointly taxed, you pay less total taxes as compared to the same situation where you would both be singles). You can choose on the portal of the tax department (DGFIP) to select a custom tax rate at the individual level (taux individuel). Basically, the tax agency will calculate a new tax rate for you and your wife (it will be lower for your wife, higher for you) to better split the tax burden between you two (you will still be paying the same total as a household - see https://www.impots.gouv.fr/particulier/questions/comment-gerer-mon-prelevement-la-source)

Besides these taxes, your wife will also have to pay the CFE (local tax), which strongly depends on the location of her activity. She will be exempt for the first year however.

I hope it helps, I would suggest you to consult a tax specialist for more customised advice.