So having into account both PrimeNumber7 and DannyHammilton answers we have these issues:

1- A pool will be paid to include a transaction with a small fee (or not fee at all)
2- A pool reserves a small percentage for low fee transactions.
3- A person mess around with a tx and do not put the charge back output into the tx. So the fees goes moon.

I'm going to deal with them:

1- That's something I've already see and asked about it: https://bitcoin.stackexchange.com/questions/93471/ive-found-two-mined-txs-with-no-fee.
But no problem: As you can see in this section https://mempoolexplorer.com/block/last/BITCOIND I can easily see the txs which has not been relayed to my node when the block arrived.
I've already discounted them in the column "Lost reward excl. not in our mempool txs" from here: https://mempoolexplorer.com/block/BITCOIND but not here yet: https://mempoolexplorer.com/miner. I'll do it.
2- If a pool reserves a small percentage for low fees tx there's nothing can be done. But I think this must be taken into the "final result" to see how much money they are spending onto this.
3- Easy: Check if a transaction deviates a lot from block average. The threshold will be high since I've seen in a regular basis tx's paying a lot more than it's needed.

I have a lot to do. since this needs more graphs and a lot of other things I need to solve first.

But I'd like somebody answer the question if running several instances in different geographical places and compare the results would be a good way to start and make the problem of block propagation time less concerning.

Thanks in advance.