On average, what percentage of total block rewards are transaction fees?

that much block reward It consists of two parts, block grant and transaction fee. The block subsidy is currently ₿6.25 per block and will be halved from 840,000 blocks in about 3 months. Transaction fees can be quite variable as they depend on the current demand for blockspace. The average fee collected in the last 24 hours was ₿0.33 per block. About a month ago, miners briefly collected block rewards with transaction fees exceeding ₿7 per block.

^{via blockchair.com}

Knowing the average fee for a block makes it easy to estimate transaction fees.

^{via mempool.space}

The virtual size of a block can be up to 1,000,000vB, so 1s/vB translates to a transaction fee of 10m₿. So here ~38 sat/vB indicates that the block collected a fee of approximately 380m₿ = 0.38₿.