Stock tune in video mode should be perfectly smooth and slow. I suggest returning all tuning parameters to their defaults. Then, before you take off, verify that you are Video Mode, and not Sport Mode, and verify periodically that you remain in Video Mode. You'll soon develop a feel for their differences.
If you wish to learn the basics of what those tuning parameters do, download the
TelloFPV manual. Nearly as impressive as the app itself, the TelloFPV manual describes in careful detail, how to use the app. On top of that, the developer has generously included practical descriptions, and definitions, of more general RC aircraft concepts, and terms. I recommend taking the time to read it like a book. To skip straight to the answers you seek, search the documnet for the relevant keyword (press Ctrl + F, and type in your keyword.) For instance, typing in "expo," and proceeding to the second instance of the word will land you on page 15, where you'll find a description of the expo function. Reading should give you a functional understanding of the concept, and experimenting/playing with the actual settings will then tighten your grasp of it.
Once you understand the tuning parameters, it will be a piece of cake to slow things down even further than default Video Mode, if that's what you are after.
One last thing. Many pilots are perfectly happy flying with on screen controls, while others like myself prefer the tactile feel of real controls. TelloFPV is compatible with nearly every Bluetooth, or OTG controller under the sun. I started with an XBox One S(?) controller, which works in either Bluetooth mode, or OTG mode (recommended,) and now currently fly using my actual RC radio, a FrSky Taranis X-Lite with USB OTG connection. You will find a lot of info on controllers, and how they work with TelloFPV, in the
manual.