Which parameters in the Bitcoin Core config file need to be set correctly prior to IBD?
To my knowledge, the only parameters that have to be set before an initial block download are the regtest and testnet options.

Anything else, including txindex, can be altered later, although it may cost you a reindex or reindex-chainstate to enable functionality like a txindex if done after the block download has started.

Other parameters such as those governing the RPC, peers, and wallets can be quickly updated at any point.