Replace lenses 1:1 or get a different one (multiple zooms vs huge range)?

Hello,

my partner and I (beginner and advanced beginner) need a new lens for our next vacation (focus on landscape and wildlife) and I have to make the final decision. I'm just super unsure.

We currently have an EOS250D and four lenses - EFS 10-18, EFS 18-55 (autofocus broken), EFS 55-250 (manual focus broken) and EF 75-300. I considered simply replacing the broken ones, but then my partner found the Tamron 18-400 and really likes the thought of carrying only two lenses while still being very flexible and I do agree. I've definitely missed shots because of having the wrong lense mounted at the time.

However, I'm the pickier one regarding IQ - a lot of photos they deem sharp enough I would delete - and I'm not sure if I'll be satisfied enough with the Tamron, though it's the end of the 400mm range where things get blurry and who knows how often we'll actually use the extreme end. Chances are also low that we'll print out something much larger than 30 x 42 cm.

Maybe this is my sign to stop being so nit-picky or to finally delve deeper into post-processing, but I'd love to get some insights from you people which option could be the better one for us (or if there's another one, budget would be around 600€, we don't mind used gear).

Thank you very much for your help!