Office 365/Microsoft365 is the best plan. It includes the local installs of the applications. The additional benefits and cost compared to the upgrading path of the "fixed" installs is less. The product and security is supported and you have added benefits with your mobile devices too.
The 2019 product has a limited service/support life and Microsoft has that already defined. Security updates usually are available 2 to 3 years past product support.
However, as you indicate you only use Word, a similar product like Libre Office may be a better alternative in your case. There are a few minor differences in capabilities, though most will not recognized/see those diferences.
The DOCX format is an Open format unlike the older DOC format.