Most people who try it and claim "vast improvements" are just kids that have no clue what they are doing, what polarization is, what a donut has to do with RF distribution, or what a MIMO array is. Which makes them just sound silly to those who do.
Bottom line: You won't be able to improve much on the antenna setup without intimate knowledge of antenna design and expensive equipment.
With an extender you will easily reach distances that are pretty much insane for a tiny toy. Long distances always come with a high risk of loosing Tello at a far distance, with almost no chance of recovery. Many Tellos have been lost because people push the limits of extenders.
If you want extreme distance records there are two options to reach distances beyond the battery range:
1) get an extender and a large derelict satellite TV offset dish
2) get a semi/pro extender. Last time I looked Ubiquiti networks had great stuff at a good price