the core wallet is complicated, can not be used for transactions before the blockchain is completed
For the record: you
can send a transaction before Bitcoin Core is fully synced. The moment an input shows up in Bitcoin Core, you can send it. The tricky part is of course that it might have been spent already, and in that case your transaction will indeed be rejected by other nodes.
those that only got involved in say 2020 will still have to wait until 350gb+ is download to get to blockheight sync of 2020+ for their input to show up..
luckily my 2012 stash pops up on GUI display in the first ~100gb
luckily my 2014 stash pops up on GUI display in the first ~150gb
luckily my 201x stash pops up on GUI display in the first ~300gb
unluckily my 2022 play amount pops up after 350gb