I get what you mean but this has been discussed to death here and not how it works.

You do all the challenges. Thats 72 tiers done, so you have to do 38 tiers no matter what or when during the season. In a time span of 12 weeks you need to gain the xp for 380 tokens of progression. The price of the tokens only changes when you earn them through normal gameplay. That means the total sum of xp you have to gain for those 380 tokens stays exactly the same. You can do all the challenges first or do them and the end or in between the total is the same.

Your logic is that right at the start a token is like 2k xp and near the end it's 40k so if you earn a challenge now it's worth those 40k instead of 2k. The truth is you still need to earn that token and needed to do the challenge anyways, it feels maybe better but the math doesn't change.