I saw a theory from a person on Twitter a while back, and part of it kinda made sense
The actual niyati/fate (what is written to happen) is D & T actually getting married and Vallabhgarh getting destroyed as a result. That's the future- it's supposed to happen, and it kinda makes sense because when Tara went back to the 21st, there was a destroyed Vallabhgarh and remnants of everyone in her kingdom (but her own), and she & Dhruv were left in the end. You can say D-T milan is niyati because otherwise, Samrat wouldn't have poisoned her brother in the first place, leading her to time travel and meet Dhruv for a cure (plus her Radha Krishna dupatta flows to him)
The Maharishi is simply trying to stop the eventual destruction of Vallabhgarh- that's his only aim.
What doesn't make sense is why Vallabhgarh would get destroyed because of the D-T milan. There has to be an external trigger, which doesn't seem to have been specified yet.
But it's good that the story is keeping the mystery/guessing part alive.
This is the best explanation and I agree with this theory that it’s destiny that DT are together and sage wants welfare of the kingdom. It is interesting that apart from the portrait nothing else of Tara was found. Krishna has joined these two so I think eventually no matter who tries to stop them - they will be. You can’t asunder what god has brought together. There is simply no way - they can create issues for them but not change the end.
One thing I can think is Tara could have saved the kingdom but as she is connected to Dhruv the destruction was imminent. So the sage wants her around to maybe change that part of past? Anyway I do agree keeping that mystery alive is the fun part.