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.

To add onto what has already been said:

Personally, my fountain pen on the go choices are durable or beater pens. I have done the TWSBI pens to school path and feel like it worsened my problems with cracking. I favour all metal pens, the nearly indestructible plastic ones like the Lamy Safari, or pens that I buy specifically to be a beater pen, like the Majohn A2.

It’s fine or extra fine all the way. In fact, western EF or smaller is probably the ideal, especially when paired with a well behaved ink that isn’t fussy about paper quality. When I was doing my undergraduate studies which required handwritten exams, my go to was a Platinum 3776 in EF nib with a well behaved and waterproof/resistant ink like a KWZI irongall or Sailor’s nano pigment ink. I would have a complete fill/top off before every exam, and then slowly use the remainder for journaling after exam season. Since it uses a slip and seal cap, I’m not really worried about KWZI iron gall being in a pen for more than a week. Doing this on a smaller budget, I would just use one of the cheaper pens in the Platinum line with a slip and seal cap, like the Plaisir. I knew I didn’t write enough in one exam to justify a piston filler and even stopped bringing a back up ink cartridge after a while.

What I am doing as an on the go pen factors into the choice of pen design. If it’s a pen I will be bringing to the office/school environment, then I will be more willing to carry a full sized pen like the Majohn A2 or the Lamy Safari as they both open and close quickly. There probably are some highly durable piston fillers out there, but I don’t have any in my collection that I would pass as beater pens.

If I’m specifically looking for a fountain pen to carry on me (bag or pocket) at all times to scribble quick notes in a pocket notebook, then it will be a pocket pen with a short international cartridge like the Kaweco Sport, the Ensso XS (my current favourite), or something of similar proportions. I am looking to minimize footprint.

To be perfectly honest though, I’ve mostly stopped EDCing a fountain pen unless I am going somewhere I know I’ll be doing some writing. I normally EDC a mini bolt action pen with a pressurized refill in my wallet and leave it at that. It’s just less of a hassle since I don’t have to worry about drying and is more flexible when it comes to signing heat-based receipt paper and carbon copy paper.

Three solutions I have tried:

• dirt cheap Chinese pens, at the time the quality control was too terrible and I had to write on carbon paper often enough that I gave up
• pocket pens, too annoying to get out of a pocket in my opinion, some people tolerate this, they end up being better as regular clipped pens to me
• retractable fountain pens, the Chinese Vanishing Point clones work great and aren’t very expensive, I’ve had one for a while, wrote a lot (15-20 refills), no issues apart from the paint peeling off, I have not tested the brass for lead