I can offer some comments here, but am not in a position to recommend a particular approach – it is up to your company to consider the benefit and risk. The optional pre-assessment on-site visit by the accreditation body (AB) will increase you accreditation costs significantly. The initial document review step that is included as part of the optional pre-assessment, is part of the non-optional initial assessment activity and fee anyway. Many laboratories do not go for a AB pre-assessment, accepting the risk of nonconformance findings (along with the financial benefit) as the document review step of the initial assessment will indicate if there are significant gaps that need to be addressed before the initial on site assessment by the AB. Even if the initial assessment onsite audit results in certain nonconformances, the laboratory will have time to address them. The only risk is possible delay in accreditation if the laboratory does not have a good understanding of the requirements for major activities. This could result in absent or ineffective implementations that are only observed as non-conformances during the onsite visit. The impact could be not being recommended for accreditation at the time of the initial assessment and having to re-apply after finishing the implementation. However, if your laboratory has given attention to the major mandatory requirements and technical competencies, then the likelihood of this should be low.
Furthermore, consider that in the current pandemic environment, many initial assessments are still being performed remotely, so it is not clear if the accreditation body would do an pre-assessment on-site visit. I suggest you engage with the accreditation body to help make your decision.
As a minimum, it is advisable to at least perform a full pre-accreditation gap assessment, including the technical scope using inhouse (independent internal auditors) or by a third party organisation or consultant, as part of your internal audit program.
For further information see the article ISO 17025 technical internal audit: The basics
The Five Internal Audit Procedure appendices Internal Audit Program, Internal Audit Checklist, Audit Nonconformity Report, Internal Audit Process Checklist and Internal Audit Report available separately from the procedure link above; or included in the toolkit at https://advisera.com/17025academy/iso-17025-documentation-toolkit/