Google Analytics working except eCommerce data

Hi,

Hoping someone can point me in the right direction, there are some very basic questions coming up!

I am looking through the Google Support site and going down a few rabbit holes!

My 2 Wordpress with WooCommerce sites were built in 2012, with a few rebuilds / refreshes since then.
Google Analytics is working fine (I think). Data is being collected anyway.

The problem I have is with conversion data, it is not being collected.
There is code on the 'Thank You' page but following a successful order, the user is no longer directed to that page.
Before I start delving into it and either setting up the redirect or pasting the conversion capture code onto another page, I thought I'd check if this was still the current method of tracking conversions?

I have heard of Google Tag Manager, is this what I need to set up now, rather than trying to fix an outdated method?

Thanks in advance