When is replacing an ERP justified?
When double entry has become structural, not when people are unhappy with features. If the same data is being entered into two systems, you need either replacement or integration. If the friction is concentrated in a few screens, improving those screens is cheaper.
What actually delays the project?
Data, not development. If the part numbering in the existing spreadsheets is inconsistent, writing the cleansing rules takes weeks. Name the person who owns cleansing, and agree the decision criteria, before work starts.
Should every module open at once?
No. Open the bottleneck axis among order, inventory and settlement first, reach real production use, then extend. Opening everything at once leaves no point to roll back to if it fails.
How should accounting be treated?
Keep period-end and tax work in the specialist package you already have, and integrate with it. Building that area yourself creates an annual burden of tracking legislative change.
How do you confirm the migration succeeded?
A row-count and total-value reconciliation table. Screens opening is not the success criterion. Migration is complete at the point where zero mismatches are confirmed in writing.