Why this is a separate engagement
Free consultancy is a sales process. There is nothing wrong with that, but it is worth knowing which one you are in, because the incentives are different.
An agency assessing whether you need a rebuild while bidding for the rebuild has an interest in one answer. Paying for the assessment separately removes that, and it means the output is written to be executed by anyone rather than scoped to suit us.
We still have a position: we work on three platforms and not on others, and that shapes what we know. Naming that is more useful than claiming a neutrality nobody in this industry actually has.
What tends to come out of it
In our experience the most common finding is that the problem is one layer below where it was reported. Slow releases turn out to be an integration design issue rather than a platform limitation. Poor conversion turns out to be product data rather than checkout. A replatform request turns out to be a theme problem.
That is the value of looking before committing: the expensive decision frequently turns out to be the wrong one, and finding that out in week three is considerably better than in month nine.
Where this leads
Often to a smaller project than the one you were considering — conversion work or a targeted rebuild rather than a migration. Sometimes to the full programme, scoped properly. Occasionally to nothing at all, which is still a result worth having paid for.