Is it possible to boost the Bluetooth signal from my iMac?

No, not really. About the best you can do is move the two devices closer to each other. You can eliminate obstacles and interference between them by moving them away from walls, metal objects, and other electronics to create a clearer signal path.

.not really, I’d recommend Sonos WiFi speakers. Many of theirs also support BT too. Good time to buy now, many are 20% off.