In my experience, brewery tours look very much alike in most "established" breweries, with tour guides that have pretty limited knowledge of the brewing process itself (e.g. they'll tell you that the mash is boiled in the mash tun, but in most cases, that does not mean that they perform a decoction...).
I've been to Regensburg only briefly, but was not too excited about the local beer scene. According to my Untappd notes, Kneitinger's Edelpils was nice and crisp. Otherwise, you've got a local Paulaner brewery etc... Meh.
I've never been to Kloster Weltenburg myself as I've heard it's massively overrun during summer, but it might be nice in October. They happen to make the only Doppelbock that I actually enjoy; guess I should pay them a visit some time
In Prague and the surrounding area,.there are so many great options that any guidance is both impossible and superfluous imho. The Czech Republic has countless wonderful breweries and many of them have some representation inside Prague. The difficulty lies in exercising some restraint during your stay so you keep your senses sharp and don't ruin your 'appetite' for beer after a day of trying to taste every svetle pivo you can find. I'm speaking from experience here...
And even if you think you already know that beer, do NOT, under any circumstances, miss out on tasting a perfectly poured pint of Pilsner Urquell in one of the brewery-owned pubs. As much as I enjoyed the beers from various smaller breweries and guest houses, that pint of Pilsner Urquell at 'U Salzmannu' in Pilsen (an entirely touristy venue, but it's a cute old building, so who cares) is always hard to beat as "the best pint I had on this trip".