vAT reverse charge mechanism or vAT exemption?

Just because your customer is exempted from VAT does not mean that you do not have to charge VAT. It only means that your customer does not charge VAT themselves and cannot deduct the VAT they paid from the French VAT administration.
Much depends on your turnover and the type of service or activity you provide. If your turnover exceeds €25,000 per year, you are not eligible for vAT exemption. For certain activities (depending on your NACEC codes), you are also automatically excluded from the vAT exemption. In all other cases, you decide for yourself whether or not you are covered by the vAT exemption.
If you select the VAT exemption, you put 0% VAT on your invoices and it does not matter if VAT is reversed. Your customer must then pay the VAT to the French VAT administration themselves via declaration 629, but 0% of the invoice amount = £0.00. The disadvantage is that if you have a lot of expenses yourself, you will have to pay vAT on them. If you make purchases from European suppliers (non-Belgian), you must declare this vAT separately quarterly via a 629 return and pay it to the Belgian vAT authorities. You can include the vAT paid as an expense in your tax return.If you select standard VAT liability, you simply charge 0% VAT on your invoices to other European countries. Your customer must then submit a 629 return to the French VAT authorities and pay the VAT to them. The advantage for you is that you can recover the VAT on your own purchases in full and only have to pay vAT on your profit margin. Please note for investments: vAT is calculated using a standard depreciation over 5 years. If you stopped your self-employed activity, you must subsequently repay the vAT that has not yet been depreciated to the Belgian vAT.