Every new pool will add in for decentralization, so that is to be appreciated. However, also anyone can decode the coinbase data regardless of the (solo) pool, since that data is exchanged via stratum. So there always was a way to verify what address is being rewarded with what amount (if the respective block wins) - and since the block hash is based on that input data, any tampering with the data will result in another hash value that won't meet the target difficulty.
Assuming on block level all is fine, you still have to always trust the pool operator to be fully operational, with up-to-date blockchain and capable to get your winning block out to the network as fast as possible. There are known and proven solo pools out there already, so as a new solo pool operator it will be hard to find trust and acceptance. In fact you can only earn trust with the count of solved blocks.
Absolutely agree to this point. Anyone can look at the coinbase transactions that a pool has set. When the miner does his work and finds a valid hash below the target, it's all based on the block header that the miner originally received from the pool. If the miner now passes the valid hash to the pool and a malicious pool operator were to try to retroactively modify the coinbase transaction in his favor, then the nonce value would not fit at all. The pool operator in this example
cannot therefore subsequently modify the coinbase transaction in order to defraud the miner.
ButWhat a malicious pool operator could do, however, would be:
The malicious pool operator first configures a valid coinbase TX and let his pool run for months or a year and hoping that many users will check it and make sure it is correct. He tries to attract a lot of users onto his pool. And after he has built up the trust of his users and gained many users he could e.g. after months or years suddenly and unknowingly changing the coinbase TX for the mining afterwards. He enters his own address as the payout address and removes the miners address. The miners would never notice this unless they check the coinbase TX beforehand for each mining process.
An even worse scenario would be if the pool operator periodically made this coinbase change according to set procedures, with the hope that users would not notice it. For example, he could use a good coinbase TX for 12h per day, and a malicious coinbase TX for the remaining 12h per day. Or at other intervals and irregularities with the goal that a user trying to check the coinbase TX of his pool will not notice anything.