I don’t know about the “weight” rating, but when I get mine filled they take about 4.5 gallons.
Also, once they are 12 years old, they can’t legally be refilled, but if you find one that’s 11 years old, you can probably get it for a good price (”It’ll be unfillable in a year - only good for scrap metal - give ya a buck for it.”), fill it one last time, and store it.
Exchange it.
The propane company that refills it can re-certify it and keep it in the stream. IIRC, visual recertifications are good for 5 years. When I worked in the industry we would scrap the junkers (broken feet, excessive rust, dents, etc.), re-valve those in need of an OPD or with leaking valves, and sandblast/paint those that started to get surface rust.
If you start inspecting various tanks you encounter, you will likely see some with hand-engraved dates in addition the the stamped manufacturer dates. That indicates they have been visually re-certified and are good for another 5 years. You might see multiple engravings on older tanks indicating they have been re-certified more than once. This engraving is more common with the larger tanks (100lbs for example) because they are generally much older than those for grills.
That's my MO.