In 2019, we traveled for 28 days to Athens, Budapest, Zagreb, Beograd, Timosuara, Oradea, and Amsterdam.
We spent $5200 for the entire trip, $200/day, for 2 people. We stayed in inexpensive hotels. We ate in local restaurants with mostly locals. We drank wine most days, and had plenty of coffees. We did not rent cars, but took trains.
How did we do this? We got our plane fares out of credit card point - we spent $2.86 for the airfare.
When I see someone say that they want to spend under $15000 for 6 weeks, this is $2500/week. You don't need to spend this kind of money. You can easily have a very nice time on half of that, and go back another time.
1) There are many hotels for $60-$70-$80/night.
2) You don't need to eat in restaurants all the time. We ate the breakfast in our hotel every day. We would then have a snack during the day, and a decent dinner, usually with wine.
3) A careful plan for each day will allow you to do what you want without wasting money.
You are younger than my wife and myself. We are 67 and 71 and plan on many additional trips. We don't plan on going to high-end hotels which are very expensive. We don't go to high-class restaurants. But we enjoy our trips.