50 feet is probably too far away. The usual rule of thumb for the kind of home automation Zigbee devices that smartthings works with is about 40 feet per “hop.“ Sometimes you can get farther, but it may not be reliable. So I think you’re just a bit too far away.
Zwave has a longer range per hop, maybe 75 feet for zwave plus. So it’s not surprising if that worked better.
As far as whether you would need an additional bridge/Gateway/hub for Wi-Fi bulbs, it just depends on the specific brand/model. Some do require an extra bridge in order to be able to integrate with smartthings, some don’t. In any case, almost all of the Wi-Fi bulbs have a “cloud to cloud“ integration with smartthings where they use the Internet to talk to their own cloud which then talks to the smartthings cloud. For example, a Meross makes Wi-Fi bulbs that have a smartthings integration and do not require any additional hub/bridge. In fact, you can use them without having a smartthings hub, just with a smartthings account. But it is a cloud to cloud integration.
I don’t know of any Wi-Fi bulbs which talk directly to a smartthings hub, that’s just not how they usually do their integrations. Zigbee and Zwave bulbs can potentially talk directly to a smartthings hub, again it depends on the specific model.
If there’s any place in between the barn and the main house where you could put another Zigbee repeating device, either a plug-in pocket socket or just another lightbulb, that would probably be enough. But otherwise you may need to switch to a different protocol, probably WiFi.
Or another alternative would be to put a separate smartthings hub in the barn and treat it as a separate location.
If you start with post 11 in the following FAQ (this link will take you directly to that post) it will explain most of what you need to know about range.