However, I'd like to know if there will be BTC miners once the supply is fully mined, and will the fees for sending from wallets to exchanges be high? I also wonder if the price fluctuations will be similar to the current bearish and bullish market. Currently, BTC is widely traded, buying at a low price and selling at a high price. But will BTC trading be possible in the future, similar to daily trading? I think this is worth discussing...

If all the bitcoins are mined in the year 2140, it means that more than 90% of bitcoin will be in the market circulating, while the other percentage might be lost which I cannot really tell. However, the quantity in circulation is enough to keep the daily market moving through trading. The difference is that transaction fees will be high and bitcoin price will be expensive.
That's a very long time in which, all of us here in the forum would have gone to meet our ancestors. Your only concern currently, should be for you to continue buying more bitcoin so that in future, you will be happy that you made proper use of this opportunity.