I did do a search and read through any posts I could find, including a fantastic trip report from David...
Presumably this one? Trip Report: Colombia, February 2024 – Glad you enjoyed it.
Yes, I think your idea to split your time between Cartagena and Medellín is a reasonable choice, though your time in both would be tight.
On one hand, Cartagena is lovely, although spending all 5 days there does seem like plenty, perhaps more than you need, given the other places you might add. Or you could easily add a day-trip or two out of Cartagena if you like - we did not do that but there are some good options. But since it's a long way to Colombia, you are probably thinking of the other stuff that's oh-so-near once you are in Cartagena.
Given what you've written above, I would probably be inclined to do exactly what you have suggested, and split your time evenly between the two cities - between the two, they check all your boxes quite well. So yes, if I was going to pick any other city in Colombia to add (at least among the places I've been, which admittedly is still quite limited), and given your time limitations, I'd pick Medellín hands down - it has so much going for it (while I'm glad we went to Bogotá, I think most visitors will find Medellín is more appealing). With just 5 days to play with, you will of course only be barely scratching the surface, but I think you can get a nice (if brief) taste of both cities.
We found that flying domestically within Colombia was surprisingly cheap, easy, comfortable, stress-free, and delightful (frankly, a better experience than flying domestically within in the US these days) - no drama, no hassles, no crushing crowds, everyone behaving. Flights between these cities run frequently and efficiently so it should be easy. The flights are short, actual time in the air is less than one hour. Medellín's airport is modern, nice, a pleasure to use. Cartagena's airport seemed older and a bit less glossy (more like an American airport) but perfectly functional.
The only downside is that you will effectively burn most of a day (and you you only have 5 to start with) with the domestic flight and the time lost changing locations. I would seek to minimize that inefficiency by looking for a morning flight, arriving in Medellín by noon, allowing you to salvage the afternoon and evening for some sightseeing.
For Cartagena, be prepared for heat and humidity. I would sleep (and spend most of my time) in the historic walled city, every street has some historic eye candy. Outside the walled city, the "castillo" (fortress) is worth a look, too (we took a taxi). The Getsemani neighborhood, just outside the walled city, is understandably popular, extremely colorful, quaint and definitely worth some wandering too.
For Medellín, I'd strongly recommend staying in the Laureles neighborhood - it's got everything you need (it's filled with trendy and enjoyable restaurants), and it's walking distance to a useful Metro stop, from which the rest of the city and surrounding hills are easily accessible - be sure to try the Metrocable system (we rode it all the way to Parque Arvi, quite a long ways). Don't miss the Plaza Botero and the excellent Museo Antioquia which is adjacent. There are large street markets nearby. Lots to see and do.
Other suggestions/recs in my trip report linked above. Happy to answer specific questions.
Have fun.
