Correction
ありがとうみんなさん。→みなさん、よろしくおねがいします。
1. We say みんな and みなさん. But we don't say みんなさん. みんなis a changed form of みな<everybody>. It's not a formal word. みなさんis a changed form of みなさま<皆様> and retains the original form "みな". さん/さま(honorific) doesn't match a casual form みんな.
2. よろしくおねがいしますI hope you will take care of me.=Thanking you in advance. To sayありがとう is strange because you haven't known whether someone gives you a favor. It might be a correct expression in English. But in Japanese, we don't say "ありがとうThank you" before we know someone gives us a favor.
Answer
There are inflection and stress in Japanese too. But they are different to English. The unit of sound is not syllable. It's mora( 拍<はく>[haku]in Japanese). Mora is similar to beat or stroke.
おばさん[obasaɴ]aunt / middle aged woman 3 syllables 4 moras
おばあさん[oba:saɴ]grandmother / elderly woman 3 syllables 5 moras
かた[kata]shoulder 2 syllables 2 moras
かった[katta]won 2 syllables 3 moras
かいた[kaita]wrote 2 syllables 3 moras
English has a stress accent and Japanese has a pitch accent. We use pitch(height of sound) instead of stress.
雨<あめ> rain: あis high, め is low. It isミ(Me/E)ド(Doe/C) if I say with a musical scale.
飴<あめ> candy : あis low, め is high. It isド(Doe/C)レ(Ray/D) if I say with a musical scale.
When we emphasize(put a prominence on) some part, we stress those place.
それは わたしのです!(あなたのでは ありません)It's mine! (Not yours) ※わたしの(です)is pronounced stronger and higher than それは.
A: だれが いきますか。Who will go? B: わたしが いきます。I'll go. わたしが いきます。※だれが/わたしがis pronounced stronger and higher than いきます(か).
If you want further information, you can take my "Japanese Accent Rules" course. (^-^)