Johanna Christina Bronckman - Who are her parents?

First of all, she was never named Thulin, - her burial: https://familysearch.org/pal:/MM9.1.1/FKRW-ZX5

Well, if you have a look at the baptism record in the DDSS for "Ulrica Wilhelminna Bronchman" (1774-03-30) you will notice that one of the witnesses were a "Fru Johanna Christina Thulin". So she definitely used Thulin as her surname.

So as a curator, what do you recommend we do? I don't have a problem removing Thulin as her surname from her Geni profile.

As long as you have primary sources for a married name it is OK to use it, - I have an example myself from the early 1800th, - 100 years before the Norwegian name law opened up for it in 1923.

Just remember that some transcriptions of sources might be influenced by the assumptions made by the person who did it.
Familysearch.org is a terrible example on Nordic records where they enter a fabricated surname on the children in Norwegian baptism records - even if the primary sources usually don't have it.

We shall always know, not assume, when it comes to names...

I recommned using the maiden name as surname and the add married names in the alias column. I do it if I find the names used or if I merge when one of the merged profiles have used the married name. It makes it so much easier to work with the tree if you see the maiden name immediately, and since many women are married twice or trice it is the only solution.

About the question itself: I note that there is a five year gap in the family´s birth of children 1755-1760 where she would fit in. Otherwise the known children are born every to every third year through time.

I would use J.C. Thulin (born Bronckman) since the sources (churchbooks and other official papers as testamonies, sentences etc) mainly, at the time wrote depending on the purpose and without any thought about what the female liked or disliked. At a baptism for a child it was important to be sure it was "proper and honorable" witnesses. To tell the title wife and to home this wife belonged was for sure a good guaranty the woman in question was not a "bad" woman.

One source is also the thombstones. We have to remember that the transfer of a daughter from one man (the father) and household to another man and household (the husband) was a transfer of money/capital/fortunes/possibilities. But also responabilities. The owning of a female and the belongings connected to her was transfered and a new houshold was constructed. So after the marriga she became the wife of a Mr XX or Mrs XX born YY. If she lost the husband there was laws and contracts about what responabilities the brothers of hers and her Brothers in Law had in the future. So compare different testamonies (after her father and after her husband) to decide!

If there are no reliable documents I use to look at what the Geni-users = relatives prefer.

As far as I have seen professional genealogist always use the maiden name, regardless of the actual customs of that time, state or culture. Am I wrong about this?

A professional genealogist add all name variants with references to time or time period and sources.

The problem is that Geni does not have such an option, so many compromise by adding full name variants to Also Known as, use Last Name for married name and Display Name for "best known as". The field birth surname should always be filled in.

As a genealogist, Private User, I Write names the way you say, but I also do it the way Bjørn say. I don't write married names in the Last Name field though, I use Also known as for those names, which is the best way on Geni at the moment to Write alternate names or names used later in life.

According to Ängelholm churchbook, no. CI:2, 1767-1834, page 140a she dies Sept. 12th 1781, burried Sept. 16th and the text is: Herr Tullnären Thulins Fru Johanna Christina Bronckman. She is 24 years and 9 months old.

I would guess she died in childbirth, because Sept. 5th their child Maria dies, 4 days old, burried Sept. 9th.

Their wedding is in the same churchbook, page 175b, Oct. 15th 1773, text: Copulerades Stads Casseuren udi Toschalen?? Herr Anders Thulin samt Jungfrun Johanna Christina Bronckman.

Anders Thulin is burried May 20th 1821 in Krstianstad, died May 12th, according to the churchbook for Kristianstads Stadtsförsamling, no. CI:8, 1807-1833, page 315a.