So when I thought of "soft cap" I actually started thinking:

Keep the 300 Myth Tome cap, but put any other accumulated Myth Tomes into a bank. Then, when the weekly cap resets, those tomes in the bank get added to your now empty weekly cap.

So say you run enough dungeons to accumulate 480 tomes. You get 300 for this cap, and then 180 added to the bank. When the weekly cap resets, the 180 from your bank moves into your cap amount, putting you at 180/300 for that week.

As another example, let's say during the course of one week, you run enough dungeons to get 900 tomes. 300 is added directly to your total and put towards your cap for that week, maxing you at 300 and putting 600 in your bank. The next week, as soon as the cap resets, you get another 300 and your cap maxes out again, leaving you with only 300 in your bank. On the third week, your cap resets again, depositing the remaining 300 in your bank, maxing you at 300/300 for that week, with 0 tomes left in your bank.

The advantage to a system like this would be A) no tomes you collect go to waste, you just don't get them right away; B) the amount of tomes going to each player remains consistent with what they have now, needing to do the same amount of content the same amount of times for what you get; C) it's helpful if something comes up and you may not be able to get around to farming your myth tomes that week, it can be done in advanced and the time won't technically be wasted.