Maybe take a different approach and try benchmarking first. You say you are new to this and at the moment your assumption is that thinking up new journeys (or palaces if you prefer) is the hard part as far as this whole exercise. At the moment you got a little less than 150 locations in total across 4 journeys.
Benchmark 1 - Pi Matrix: you will be asked to give the 5 digits to the left and the five digits to the right of the 5 digits you are given within the first 10,000 digits of pi. People store this as 5 digits in 2,000 locations. Roughly 10 to 20 times more the number of locations you have atm.
Benchmark 2 - Memory Championship: At a national level 10 events between 5 and 15 minutes each. Some events have 2 or three trials. Assume that for a decent beginner’s score (1,000 - 2,000 points) you’ll need 20 locations per 5 minutes exercise. In total this will be about 300-400 locations.
Benchmark 3 - Int’l Master of Memory: similar to benchmark 2 but instead of 15 minutes for the long events, it’s 1 hour. You need to get 1,000 digits, 10 decks of card and a deck under 5 minutes for that; plus, your overall score has to be 3,000+ In total this will be about 1,000 - 1,500 locations.
Now, the only difference between you and those people is that they actively compete in memory sports; however, they do NOT have seen more places in their lifetime than you have. Also, that’s probably not the only palaces they have, so make of this what you will, but your number of palaces can easily at least double or triple.
All that being said, if you do long term memorization, you have less of a problem with “ghosting” when reusing your journey. During competition it’s almost impossible to reuse a journey, because it needs a couple of days before the images fade… but you don’t even want your images to fade.
Imagine a bicycle, in order for the next person to get on the first person needs to get off the bike. That’s the short term, recall and forget after… really, exam cram kinda attitude during competitions. For you (and the pi matrix) it’s more like a bus full of people.
It’s hard to mix up information on the same route, because the stories are distinct. Consider watching 3 tv shows you like each week… do they mix together only because they all happen in the same week… no because the actors and stories are different and you can follow all three at the same time. Now, do we all prefer binge watching a whole season on Netflix instead… of course we do, but that’s how the generation before you managed to life their lives too.
Bottom line, yes put two stories on the same journey if you have to… maybe assist it with suggestions from above, like the second story you walk the locations back-to-front; but you are really nowhere close to running out of ideas yet as far as new locations / journeys.