A big question is specifically what 'larger storage machine' is referred to. From what I understand, at least some NAS can be set up for remote online access. If that is the case, one could in theory have the main photo library on a NAS at home, and access those photos from out in the field, continent upon having line access (perhaps via a smart phone).
Synology for one (of likely many) can definitely do this. And it can be configured as a free personal cloud for easy access to files like photos from anywhere.
If the "larger storage machine" is as BLtheP just considered- a Mac Mini- OP could:
- attach all the external storage needed to the Mini,
- install the "too big" photos library on Mini storage,
- prune the laptop photos to the best ones (for always with OP),
- set up remote access to that Mini and-
- when OP needs a photo(s) not in the "best of" with him/her, access it on the Mini remotely and upload it to free iCloud or Dropbox or similar space on the spot to then download to the MB in the field.
This would be a workaround to having to pay some ongoing rent for cloud space to store the photos.
There are also those "lifetime" plans from select cloud storage options. Spend a few hundred and buy 500GB or more space for "life*" While I doubt life* will equal an actual lifetime and OP may not want to trust total strangers with their photo collection, that is also a relatively cheap way to have access to all of them without a monthly rental.
On a quick search just now, I see 500GB for $200 lifetime* from one... and there are occasional deals offered around here and other Mac fan sites that pitch lifetime* cloud for fixed fee amounts. A concerted effort to "shop around" would probably do better than that. I wouldn't trust the cloud with the
only copy of the bigger library but that could be an option for OP if they want the special deal and access to all photos without buying a Synology, Mac mini, pruning, etc.
If was moved to do this, I'd buy a big hard drive or two, backup my big library on a couple of drives I control and then upload them to strangers in the cloud. Then, if lifetime* actually turns out to mean when they run out of cash flow and suddenly shut down with no access to anyone's files, I still have my big library backed up at least twice on drives I control.
And if the photos are important to me, one of those drives I control is stored offsite to protect against fire/flood/theft scenarios.