First of all, this is a very late answer but as there are no answers I guess a late one is better than none at all. Further more I can’t really tell you why using fountain pens outside your house feels off to you. But I have used them outside (at school) so here are some tips:

Good seal. Having a pen that seals well is crucial. If your nib/feed dries out it won’t write immediately. At home this is not a super big deal, just get it going at your own pace and write. But if you are out and about and the teachers says something important and you go to write it down and it doesn’t work immediately, this is a big issue. I used a random less than ideal pen that would have a shitty seal and it wasn’t very good at school.

Not too frail, not to precious. If you don’t have your pen ready to grab, you simple won’t write with it much. For me, I just throw my pen in my bag. If you have a pen that is expensive or easy to damage, you probably don’t want to do that. Maybe you’ll put it in a sleeve or a case. That extra step to get through before writing with it is not a big deal at home, but on the go it doesn’t promote the use of the pen. Say you have one pen in a pen case and one ballpoint just right in the bag. You quickly need to write something down. You would probably quickly grab the ballpoint.

Paper compatibility. At home you say you write your journal. You might have a nicer notebook for that. On the go, you might encounter some shitty quality paper. Maybe someone gives you a form to fill out or something. A pen with a beautiful wet nib gushing ink might be amazing at home but on cheap paper it might turn out unreadable. The pen might run out quicker and it will take a long time to dry. Going with fine or extra fine nibs usually helps with these things.

Leaking. This is a non-issue with most modern pens but some vintage flex nib pens have an amazing feed that just keeps going. But they also casually leak ink if you shake the pen. So an old Waterman might be amazing at home (I want one lol), you could regret taking it outside.

Ink capacity. At home, running out of ink is no big deal. Just fill it up, I kinda like filling up my pens even. But on the go, you might benefit from a bigger ink capacity. If you are unconsciously afraid to run out of ink, you might not write with it much. But if you are confident that you have enough ink, you will be happy to write with it whenever you have the chance. This also depends on how much you write. If you write whole pages every time you go out, a standard converter or cartridge may be completely fine for you. On that topic, some people prefer cartridges or inks that are also sold in cartridges for their on the go pens. This is because if they run out, they can pop in a cartridge on write on.

Size. If you throw you pen in your bag, this is not a big issue. I just use regular sized pens outside my house. However, you want to carry a pen in a pocket, you might prefer a pocket sized pen. After all, if you don’t end up carrying it you won’t write with it.