I've heard that Bitcoin will finish mining in 2140, which is certainly a long way off. There's still a chance for us to continue dca BTC, as there's a strong possibility that in the future we'll see a higher BTC price than today, as BTC is constantly being mined, and we'll eventually see the BTC supply dwindle. Currently, I know there's only a small amount of unmined BTC left, approximately 1,100,000. While the number currently mined is 19,900,000, there's still time to accumulate BTC.
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...

The answers you are going to get are going to be opinion and believes and not really what miners has in mind. One thing I know is that nobody can actually predict what is going to happen in the nearest future but miners are not going to abandoned Bitcoin because the majority of miners with the largest pool hold more Bitcoin than the rest of individuals except for large companies that are spending billions buying Bitcoin every day but miners are getting their own from their effort of Bitcoin support.
No matter how expensive Bitcoin becomes, the transaction fees might not be really that enough to compensate miners like the blobk rewards they get on an average, in fact the fees are even getting smaller now even as Bitcoin is pumping all this while. Transaction fee are as low as 1 sats/vbytes and highest is 10 sats/vbytes for days when there is spike, if you compound this together in a block, the fees combined wouldn't yield anything but I still believe that miners are going to be here until the last days of life with Bitcoin.