The ETA feature is like Bitcoin Core's floating fees which estimates the number of blocks it takes before the transaction gets included. It's more expensive than mempool as it takes a more conservative estimates based on the recent trend with the mempool and/or the fees paid by the transactions included by blocks. Mempool is a more up to date method to estimate the fees as it takes the current conditions as it is; each blocks takes 1vMB of the top paying transactions by fees/size and having a transaction with fees that puts it under that 1vMB threshold will probably result in your transaction being included in a block if it a block is mined immediately. Mempool is not the most accurate as it cannot take into account the possibility of blocks having a longer interval, etc.

2700bytes is fairly big. You likely have a lot of inputs which results in a larger transaction. A typical 1 input and 2 output bech32 transaction is about 142 (virtual)bytes. You'd want to consolidate[1] your unspent output often to have a smaller transaction size.

[1] https://bitcointalk.org/index.php?topic=2848987.0