Trunk setup procedure - review?

Your plan is fine. The easiest way to do this without major impact would be to create the trunk with two new unused interfaces, replace the interface on the VLAN with the trunk, and test. This also gives rapid fallback if a failure takes place.

If this is not feasible, likely due to lack of available ports, then I would suggest moving the second interface into the trunk before you do your testing. Link aggregation is unlikely to cause issues, but if you have something misconfigured on the switch or the BigIP, you will want to know this sooner rather than later.

Make sure that you have LACP configured properly if you are using LACP. It doesn't hurt to have both endpoints active, but having both endpoints passive will bring your trunk down in short order.

I would be aware of the contents of this article: https://support.f5.com/csp/article/K1689