OP is typically relating the high fee to exchange withdrawal fee which is not the right estimate of bitcoin transaction fee because exchanges charges more than usual.
Also, what I have noticed to decongest the mempool more is the bear market when the price of bitcoin decreased, some people will sell their bitcoin at the time and bitcoin transaction will be reduced unlike the bull market that transaction will increase and the mempool will be congested just as you commented.
In the first paragraph, it is worthy to note as you stated that the custodian exchanges fees tends to be higher than non-custodian use because they are in business. Instead of them to have two separate fees(transaction fee and service fee), they incorporate them in one and call it transaction fee.
In your second paragraph you said of decongestion of the mempool because of bear market. I have a question; either people are buying or selling bitcoin, there is transaction in both cases. That is to say, if people are rushing to sell off their Btc during a bear market, same congestion that is experienced during a bull market should also happen. Or am I missing anything?