This is one tricky problem i tried to "fix" indirectly by searching for alternate ways to (at least!) detect exact time/turns. Don't forget most AI tend to "rush-build" stuff around if their production stash allows it. Wonders sometimes are quickened too during their last one or two turns.. seen it often enough through the means of smallish personal file edits shown below.
As you explained above.. currently, the only reliable method to obtain nearly accurate hints is to watch any of those 3D/Modeling progress with help from a custom "coloring" schema provided by 1) the MoreLenses mod & 2) A sharper FOW layer that enhances all map elements at once.
The FOW.artdef file has to be edited though.. like this on line #235;
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<Element class="AssetObjects..FloatValue">
<m_fValue>0.050000</m_fValue>
<m_ParamName text="Models_ParchmentWeight"/>
</Element>
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BigBen is the toughest to accurately predict since its last clock "beams" construction take three turns on average **IF** you could catch that final process at the right moment to react & beat the AI to it. Needless to say.. there's a lot of luck involved for every attempts, btw. You have to synchronize your rush pace with spare time against competing AI(s).
Except for another mod created for Civ5 (adeptly called -- Wonders Race) that kept track of simultaneous projects & giving some warning -- Civ6 still doesn't have a specific mod that does it for us, sadly. I wish i could code that out from the afore-mentionned C5 LUA source code version -- it shouldn't be that hard for somebody with the right skills, i guess.