AIUI, the OTS proposal is in another delay until Sept 2024
https://www.ofcom.org.uk/news-centre/2024/enforcement-programme-into-failed-implementation-of-one-touch-switch
I think the providers are supposed to be following the aims of the scheme already although the mechanisms for making it happen, as envisaged, do not seem to be running yet.
From many past topics on here, I think you are correct that your existing line with VM must be fully operational, and have no pending disconnection request on it, to be able to port it.
I no longer have a landline and do not use VOIP but some of the regulars on here have moved to a third party VOIP service for the reasons of flexibility which you have suggested.
It is supposed to be the gaining provider who leads the porting process and deals with any/all issues with the porting.
Have you checked who owns your landline number? Is it VM? You can check here
https://portal.aql.com/telecoms/network_lookup.php
In the 'gone wrong' number ports described on here, the customer's phone number has typically been ported before to VM (so another provider actually 'owns' the number). If something goes wrong with the port, and the number becomes 'lost', it goes back to the range holder of the number. There are supposed to be 'rescue' procedures in place for a lost number during porting under OTS but implementing them seems to be erratic in the topics described on here.
Mostly porting issues on here happen when VM is gaining a number (and the problems there typically involve the customer cancelling their existing service before the number has ported and/or VM delaying starting the new service and/or the range holder of the number being a third party).