I am currently running 5e but do not have a bag of holding in the party.
I would generally not sweat it until the party tried to shove a ton of big things in. I have never been a fan of encumbrance, even in B/X days with things listed out in coin equivalent encumbrance for each piece of equipment in the basic set.
I vaguely remember an OSR system, maybe Black Hack, has items measured in encumbrance slots and that could be adjusted for your simplification but with system desire here.
I also generally do not sweat things like mundane ammo so options like ammo dice (a similar system) are not something I have picked up.