Just wondering if there has been some research done for when the block reward becomes irrelevant.

We can do some basic math like as long as bitcoins purchasing power doubles every 4 years it should be enough to secure the network. But I'd like to know in more detail about the upper/lower bounds of sending an onchain TX. 100k per TX could be pretty reasonable with 2-4BTC reward per block. 1m per TX with 20-40BTC reward would probably not be sustainable.