You will most likely need both smarter heuristics and better deductions for larger instances. As mentioned, I've seen well-engineered custom tight C++-solvers with decent heuristics and the normal deductions take >1 hour on some instances. Most cases I've tried can be solved reasonably quickly when using OR-Tools CP-SAT which uses constraint programming style lazy clause generation to a SAT solver and custom MIP relaxations in a portfolio.

For a list of interesting instances to test, see this file: https://github.com/Gecode/gecode/blob/release/6.3.0/examples...

I should probably do some benchmarks on that with the standard MiniZinc solvers.