Here is my conclusion:
During the Sefirah days (the days between Pesach and Shavuos), the Poskim differ if one may perform a siyum sefer torah with dancing and music, it seems proper to avoid unless there are particular circumstances that preclude the celebration being done at another time.
Certainly on Lag B’Omer one could make a siyum sefer torah since it is a day of celebration. Likewise a few days prior to Shavuos one may make a siyum sefer torah.
Shulchan Aruch Harav (Orach Chayim 493:1) holds that one is not allowed to dance [nor play music] even for the sake of a mitzvah during the sefirah days. (See however Hiskashrus (vol. 246) which understands the view of the Shulchan Aruch Harav differently to even allow making a siyum sefer torah during the sefirah days.) See also Piskei Teshuvos (493:5) which brings that one who does make a siyum sefer torah should at the very least avoid playing music.
According to the Keren LeDovid (Orach Chayim 119) one may make a siyum sefer torah during the sefirah days. This is because according to the Magen Avraham (493:1) as explained by the Machatzis Hashekel the only problem with dancing is when it is not a mitzvah dance, however, when it is a mitzvah dance, one may dance even during the sefirah days. Since that dedicating a sefer torah to a shul is a mitzvah, thus it is allowed to have a siyum sefer torah together with dancing as well.
Prior to Shavuos even the stringent view may make a siyum sefer torah since many Poskim hold that there is no need to refrain from dedicating a sefer torah during the entire sefirah days. Additionally, dedicating a sefer torah close to the holiday of the giving of the torah is an auspicious time.
I would appreciate your input.