"schätzungsweise" und "ungefähr" sind Synonyme.
Aber in "schätzungsweise" steckt das Wort "die Schätzung" (estimate).
If you are "gespannt auf etwas", you are, of course, also (very) interested in something. You are also excited and expecting. It is a strong feeling in positive anticipation. Sometimes it's also used to express irony and disbelief.
If an unreliable (as it is well-known) person promises something, you can answer: "Da bin ich aber mal gespannt!" Instead of: "Actually, I don't believe you. But there you go."
"Meinetwegen" has different meanings.
If you use it as a single word (like "aha"), you mean: "I've got not opinion." Or: "I don't care."
Even when it means "if you like", it sounds far less optimistic than "I agree".
In a complete sentence it is often used to emphasise your personal opinion and/or an approval, to express that your words shouldn't be understood as a rule.
"auflegen" is mostly used for music, for phone calls and for publishing.
Music: Der DJ legt in der Disko eine Platte auf.
Phone calls: Nach dem Gespräch habe ich (den Hörer) aufgelegt. (Although modern telephones, mobile phones and smartphones do not have a receiver to "hang up" anymore, we still use "auflegen" when stopping a call.)
Publishing: Der Verlag hat das Buch neu aufgelegt. (The publisher brought the book out again.)