The choice is not really theme versus custom. It is how much customisation a theme can absorb
The decision is a spectrum, not a binary — stock theme, configured theme, customised theme, and custom build are four different commitments.
Most stores that think they need a custom build need a customised theme, which costs a fraction and keeps platform updates.
The real cost of custom is maintenance, and it arrives every year rather than once.
Horizon's nested block system widened what themes can express, which moves the threshold further toward themes than it was.
For most stores, yes, particularly with some custom work on top. Premium themes handle the standard commerce patterns competently and receive updates.
The full guide has the formats, the deadlines and the worked examples.