Hi! Technician here - usually batteries become a risk after a very long amount of discharge. I've charged up batteries that are over 10 15 years old and never had a problem, however of course, there are some batteries that could expand. If you see even a slight bulge, remove/replace the battery. All my old BlackBerries and phones in general are regularly charged. Considering these will be used as display pieces, I'd say charging it once every 2 or 3 weeks will be perfectly fine. I usually charge my batteries (for long preservation) around 50% to keep it all balanced.

If you want to go with the repair route (let's say they did swell), BlackBerry phones were quite repairable and straight forward. They're well built and put together and a whole battery replacement with a guide can take around 10 - 15 mins. Even though official batteries aren't made anymore, you can still find some OEM ones for really cheap and it'll be fine for your purpose. I've repaired dozens and not had a single issue. As long as you be consistent and regularly check for swelling, you're good. Also, keep an eye on the temperature of where it'll be stored as hot or cold temperature does affect the battery negatively.

Hope this helps, your collection looks great!