TBH whilst I’m taxed in France my flying is done elsewhere so I can’t tell you how the fisc handle French airport based pilot’s sector pay, etc, but given the current system for assessing and charging tax/social charges often works retrospectively I do understand why you need an answer...but given how important this is I'd not rely just on this forum for the answer ..I'm not sure how well "but a guy on Pprune told me..." translates into French or how well it will go down at the tax office..
Have you tried asking this question on the French sub-forum? Have you got membership of one of the French pilot syndicats such as SNPL? They might be the places/people to ask initially.
Failing that I can only reiterate that your best bet might be either an accountant who understands aviation or seriously consider asking at your local tax office - and that really wasn’t a joke when I mentioned it earlier: there will be on office local to you ( you'll find it in the Pages Jaunes or no doubt with Google), unlike many other countries you can walk in off the street and ask at reception for advice, and I've always found the staff helpful though sometimes a bit formal...but the front desks and offices will be very busy over the next month handling tax return queries so you might find you are dealing with less stressed offcials and shorter queues if you give it a month before walking in..
However you do it, Good luck getting this sorted out....