The underlying OTBR software is good, and is used by several commercial border routers (I heard that Eero uses OTBR in fact). The HA UX for Thread setup is still lacking, but it’s not entirely their fault; the Thread protocol in general is missing some interoperability features and is fairly new on the scene, so UX will take time to improve. Notably it is one of the few (only?) TBRs that let you import a credentials dataset so, in theory, you could have just added it to your old mesh and then turned off SmartThings without re-commissioning anything.
The ZBT-1, on the other hand, has had a number of problems. And while I haven’t seen delays like yours with Thread, I have seen them with Z-wave almost always due to mesh/signal strength issues, so perhaps moving your usb hub closer to your lights, or adding a repeater in the space between them, could improve things. You can even make a repeater yourself for about $5. Alternatively, unless your goal is to get rid of the SmartThings device, you could also still use it as a “dumb” TBR. Remember: functionally, Thread is more analogous to WiFi than to ZIgbee — Thread Border Routers are just network devices that move packets between subnets, they don’t do any of the “smart” control.