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.