Yes, self custody in the way we know it will never scale and will also exclude the majority of the world’s population because of fees. There are ideas on how to maybe share UTXOs among several people and that may help. But I don’t know much about it.

If you imagined that the whole world will be able to use the base layer then Satoshi’s creation is probably flawed. But people are working on layers on top of Bitcoin to increase throughput and lower fees.

Fees will always fluctuate but if they will be as low as they are now in 10, 20, 30 years time is highly doubtable. When people share UTXOs they share fees too, so that might help. But what the fees really are we can just speculate. Fact is block size is limited and if more people want to transact they have to bid to get into a block. That let’s fees rise.

To DCA $20 and always go send them directly to your cold storage is a bad idea. Every time you do that you create a new UTXO. Those are going to be worthless one day. It’s much better to use a good exchange, accumulate on the exchange until you have like 0.01 BTC and then send it to cold storage. At least in my opinion. And consolidate those that you have already sent as soon as fees are low again.

People will have to use block space efficiently, yes.

I don’t know about your theory with miners.

But yes, miners choose usually the txs with the highest fees.

Don't know if that helps you