Yeah, around level 30 is when you need to start tapering off. It really depends on how early you get a level 3 and what your best towers are given your current elemental configuration. I feel like good level 3 towers like Nature/Water/Fire might be able to last you deeper into the 30s.
One important thing I didn't show off in the video because it was supposed to be in the second half is the idea of "prebuilding". I'll update the OP with this info, but basically, you build a block of towers at the start of a wave like normal, and then immediately start building a bunch of level 1 elemental or level 1 dual towers in the area that the creeps will be once the interest timer ticks. This way you can sell your DPS core right before the interest goes off and immediately finish upgrading the "next core". You obviously will lose some gold on the next interest tick because of the investment, but it saves you so much time and you still get close to the full amount, so in the long run it makes up for it.
Eventually you're going to want to set up a few staple towers that you never mess with, like Well/Blacksmith/Enchantment/Polar/Slowing towers. With your core of support towers down, you can afford to stretch out the sell/rebuild cycle a little bit longer. By the time you hit level 50 or so (probably before then), you're gonna want to just play like normal.
