What are the objectives?
KISS keep it simple with Wordpress and you should be able to find plugins that do what you need.
Maybe you’re excited to use something cooler like nextjs or whatever, but consider they might need someone else to maintain this in the future so choosing a bespoke or unusual solution might be the wrong choice even if it has better features from a technical perspective. Maintainability and the ability to hand this off to someone else is very important.
News, events, mailing list, perhaps even a registration form etc.
Make sure it’s responsive since mobile will be a common platform. Make sure it loads fast, perhaps use a CDN or caching plugin.
The number one mistake I see almost all union websites make is that the design is clunky and there are too many links / options on the initial page.
Have a clear call to action to join.
Have a “members” section for resources.
Keep the landing page simple with the most important actions & purposes only, probably “join” and the current top campaign only or a link to a members zone maybe, depending on high priority purposes. Less is more.
Have a blog feed for news, updates, campaigns, and this will help with SEO. Link to other unions maybe and ask them to link back to you, syndicating each others news with link backs. But this news feed should probably not be the home page so the home page can stay clean.
A good approach is to start listing the necessary functions of the website and rank them in groups. Priority 1 items only should feature on the home page and this should be a small group. Priority 2 can be more expansive, and accessible via a menu on the home page. Tertiary priorities can be linked elsewhere, eg within the “members” page you can have links to important documents.
Have a mailing list, probably several, one for important member announcements and another for news, maybe another for campaigns & events. In the sign up page they could join all 3 at once but allow users to only subscribe to the most important feed only if they want since you don’t want to lose subscriptions due to people being uninterested in news.
Probably you can use mailchimp for news letters.
Keep it simple. Start with the prio 1 features first and get them functional then ask for feedback from users. If possible, find a focus group of union members you can use as a ready-made test audience to make sure they understand how to use it.
Consider your audience. Older and non-technical people need simpler designs. Design for your elderly grandma who doesn’t understand what an iPad is.