PReDS
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As someone who has played approximately 20 games on Jebb in the 600+300 format, both in tournaments and practice games, I would like to note that it is advisable to accumulate some time during the first week to allow for a more effective push for the Gate of the Worlds (GOTW), otherwise you might indeed lose a day for no reason.
An alternative option is to not rush the GOTW, but rather to attack it towards the end of the turn, so that the timer that ends after the battle does not interfere with development.
A strategic nuance is that it often makes sense to play turns 116-117 purely symbolically, not trying to gather everything that the army of the first week gathers, but instead saving the timer, setting up chains, so that on turn 121 you can quickly complete everything. This will result in reaching the center one day later, but it will be much more stable and with a larger time buffer. In particular, the lost day is often compensated for by the ability to move after pushing for the GOTW.
I think that all of this is also applicable to 1000+400.