Do it yourself when the setup is close to out of the box, one person has the time and inclination to own it, and getting it wrong is cheap to correct. Bring in a partner when the design has to reflect genuine process complexity, when data is being migrated, or when a failed rollout would cost you more than the implementation would.
Plenty of businesses run excellent Zoho setups they built themselves. Telling you otherwise would be selling rather than advising.
You do not have to choose one or the other. A common arrangement is a partner for the design and structural work, then your own team for ongoing configuration once the foundation is right. That keeps the expensive decisions with people who make them regularly and the routine work in-house.
If you read the first list and recognised yourself, start on your own and call someone later if you get stuck. That is a reasonable plan and we will say so.
If you are unsure which list you are in, describe the situation on a call and you will get a straight answer. Book a call.