I want to keep Dexxter as a backup to my standard accountant. Is that a good idea?

Not really, unless you only keep your accountant to fill in your taxes, I don't see any advantage in that. You would then have to book everything in the same way as your accountant, and there is a good chance that there will be some differences.I do all my bookkeeping myself with Dexxter, but I visit my accountant for an hour every quarter to check that everything is filled in correctly. It's not necessary, but I find it useful to ask him any questions I have. I do the same with my tax return.

Thank you for your answers.
I am concerned that I have to pay so much to the accountant, among others, in relation to my net profit. I am really just a small sole proprietor (dog/cat groomer). With the costs of social security contributions (which are of no use to me because I have no rights) and many other costs, I am even considering stopping...
Personal income tax is also a difficult issue. I have read the reviews about Tax-on-Web and they are not positive...

I find the advisors at Dexxter very helpful. I have been in contact with one of them a few times and they can check everything for you. You have to be careful, because some of them don't do this because they just want to take over your entire file. But Tom from https://tuah.be/ was recommended to me by someone from Dexxter, and he was really good able to help me. He is also very transparent about his prices. Thank you. I have requested a quotation from one of the Dexxter advisors. Let's wait and see what comes of it.