I don't think you can assume that a full licence = competence. You can arrive in NZ on pretty much any licence from Western Europe (think Greece, Italy, etc), and swap it for a NZ licence within 3 days with no test required. Also, if you passed your test 30 years ago, you could have had 30 years of practicing your really bad driving habits, so much so that you are an excellent bad driver!!

We use an online assessment from DT Driver Training and that gives us a list of things we might need to focus on for each driver - gaps in knowledge, plus risks inherent in when/what/where they drive. Then we get a driving instructor from the local area, or we do some classroom or online training to fill in the gaps. Mostly it's around low-speed manoeuvring because that's where the damage occurs.

If we have a new vehicle, we do an induction on the vehicle, take them through the main features and any differences between what they've been driving and what they are about to drive.

There's virtually nothing about towing in anything you have to do for a car driver licence, which is surprising given how many people tow boats, caravans and the like in NZ.
We put our drivers through DT's online driver assessment first because it covers off a lot of scenarios. The output of that gives us a plan for where the gaps are. We then use a mixture of in-vehicle assessments at TR Driver Training, some other online training with DT, and some in-house instruction/coaching. We found this is a good targeted, efficient way of ensuring we understood the risk profiles, and maximised our training budget and schedules.
Towing is a significant risk because the issues include coupling, load security, defensive driving and low speed manoeuvring. A mixture of in-house instruction around our SOPs plus some online training covered off what we needed.

The question is really whether online training is a substitute for in-person training. In our research we found that online training is better for theory than in-class (better retention and understanding), and even better when people have English as a second language or learning difficulties because they can take it at their own pace and use language/literacy tools. That's generic across whatever topics, not just specific to driver training.
We use www.drivingtests.co.nz for things like road rules, fatigue management, logbooks, load security, etc. Then we put selected people with TR Driver Training for in-cab or in-car assessments, plus any other specific machine-related assessments. Online also means that we can maximise the time in the vehicle because DT's courses are fairly cost-effective while hiring a trainer is expensive - we don't want a trainer standing in front of a whiteboard explaining stuff at $180/hr for 4 trainees when the equivalent online course works out at a third that cost, and they can go through it time and time again if required.
What we found didn't seem to work was the video-based hazard perception stuff that's out there. It's just not realistic enough and the drivers found it very frustrating. That kind of thing seems to be better with a trainer. One of the team is a member of the Institute of Advanced Motorists so he helped with hazard perception - using Roadcraft (what the UK police use - you can buy it from Amazon) we started identifying what things drivers were missing, and that was supported by some of the online courses. We encouraged drivers to do 'commentary drives' where they described what they were seeing and why they were making specific decisions; that seemed to make a difference because it made them mindful of their driving. There are plenty of YouTube videos about how to do that, or a trainer should be able to help.
In the end, the blended approach worked for us - theory online, practical with a trainer, then put selected people back through the theory (mainly because you forget 90% of what you learned within a couple of weeks).

We have D endorsed drivers and the 'legal' requirement is a 5-yearly refresher which is too long. We make them do some online training at roughly 2.5 years. As Craig and Darren say above, you should choose your own intervals based on risk and how competent you think your team are.

We use the same DT course as we have multiple operators and truck drivers with English as a second language and they can take it at their own pace to make sure they understand it. Before then, operators would come back from classroom courses and we could tell that they struggled to maintain enough focus to pick up everything that was taught. There's a Tagalog language option, so maybe worth giving it a go in your scenario.