Hi Scholar,
<<Absolutely, a 'great tribulation (without the definite article) would foreshadow 'the Great Tribulation described in the next pericope, vss. 29-31 and 'the great tribulation' Rev.7: 14>>.
No, you can't have it both ways. The tribulation (without the adjective great) of verse 29 is referent to the unrepeatable great tribulation of verse 21 which could not foreshadow a greater tribulation.
21 For then shall be great tribulation, such as was not since the beginning of the world to this time, no, nor ever shall be.....
29 Immediately
after the tribulation of those days shall the sun be darkened, and
the moon shall not give her light, and the stars shall fall from
heaven, and the powers of the heavens shall be shaken:
Note Rev 7:14 is also about a "great tribulation" (without the definite article)