Basically, call first! Some agents will tell you on the phone to please fill out the contact form in writing first (and yes, they want to know your phone number, among other things), but this way you already get some information and get yourself into the game. Anyone who "hesitates" and wants to correspond back and forth for a long time has lost. Good properties, at least here, either go under the table immediately = to relatives, acquaintances, work colleagues, clubmates, etc., or in the rare cases where the sellers don’t live locally and word of mouth doesn’t work so well, they are online for a maximum of one week.
Sending an extensive introduction of yourself by email (which is not even asked for), while at the same time not leaving a phone number, doesn’t really fit together.
Am Mittwoch Abend haben wir dann Unterlagen zum Abschluss eines Maklervertrags erhalten lediglich mit der Information, dass wir erst nach dessen Unterschrift weitere Informationen erhalten.
That is quite common. Before there is further info like the address, etc., you have to assure that in the event of a successful purchase (and only then!) you will pay the commission. This is not yet a valid contract from which payment claims arise.
Wir haben unser Interesse erneuert und auch erneut um Erhalt von Informationen über das Objekt, Erhaltungszustand und offensichtlicher Reparaturbedarf, verbaute Materialien, Fertighaushersteller etc. gebeten.
You could have saved all that for the viewing appointment. Too early at this point.
Wie ist denn ein übliches/sinnvolles Vorgehen?
What is usual often simply depends on the location (and the desperation of the buyers). In Munich, interested parties sometimes outbid each other already at the first (mass) viewing. Whether that is sensible is another question.