Do you know if this Helmut will be in operation during autumn?

Operations of specific historic trams are only announced some days in advance (same as in Kraków or Wroclaw). You should contact KMPS, sometimes they can foresay also earlier... however, changes are always possible. (For last sunday KSW was announced, but its motor has burnt down, so N was operated instead...)

Thank you! Could you please advise where else not so far from Ostrava I can find GT8 and similar vehicles?

I know about Lodz too, but seems the season is ending in two weeks already so I will not make it probably. Also there should be Helmuts in Grudziadz but it is everything it comes to my mind.