Regardless, would moving to HFSC make sharing/borrowing between interfaces easier?
Shaping can't work across interfaces, but if there's a way to get two or more interfaces to bridge to a pseudo-interface, and assuming you can shape that pseudo-interface, you could probably do it.
When you shape an interface, you shape the data leaving the interface. Shaping your LAN interfaces effectively slows how quickly you can download. With the naive setup for multi-LAN, you can't say how little bandwidth each gets, but how much. If you have 10Mb to split, you may give your guest 2Mb and your main LAN 8Mb.
You may also want to try enabling CoDel on the child queues. If you have less than 1Mb/s, you may not want to do it. CoDel seems to have issues with 1500MTU with bandwidths less than 1Mb.