[...] estimate the enthalpy of formation using the enthalpy of combustion value, which is available. [...]
Which is the standard method from which enthalpies of formation are obtained and published.
Using instead the heat of hydrogenolysis should give more precise figures because hydrogenolysis releases less heat than a combustion, but I suspect it would be impractical, or at least less general than a combustion.
When a molecule like di(2-ethylhexyl) phthalate has no tabulated enthalpy of formation, the most accurate way among the many I've tried is to compare with other molecules which are tabulated. If (I didn't check) you have enthalpies of formation for dibutyl phthalate, butane and 2-ethylhexane, all in the same state of aggregation, then you can make formal transformations and you get reasonable estimates.
Though, that last way is less systematic hence more difficult to teach. It also fails on cage molecules - no method exists for them.
The biggest single difficulty, whatever the method, is to guess whether a published enthalpy of formation is measured or false.