Automatic speed drop on my TP Link Archer C20 v4, Please Help

Roaming is predominantly a client side function. That is to say, your phone, laptop, etc. will roam based on the condtions it sees and the algorithms/methods implemented on those devices.

There is 'classical roaming' which is entirely on the client. For this to work well, you must tune your APs such that the radios are using non-overlapping channels and usually reduced power levels such that you can cover the necessary area while minimizing the amount of overlap. I like the Crosstalk Solutions wifi optimization video as a good explainer of how to achieve this. While he does talk about the Unifi products, it applies to all wifi configs as long as the features are exposed to the user (and they are in OpenWrt).

Beyond that, you can add other methods to 'enhance' roaming -- 802.11r (as well as k and v) and/or minimum RSSI values to kick clients off the network can all be used for this purpose. However, they are predicated on a setup that has already had the 'classical roaming' tuning (first priniciples), and these standards and methods may, in general, cause other problems because some client devices just don't play well with these approaches (even modern devices may not be happy). Personally, I don't use them and I don't recommend any of these methods unless there is a demonstrated need. Classical roaming can be nearly seamless when done properly.