Hi @CrucialNic,
just found this post and thought I add my thoughts here. I totally understand what you are after working with WP over a decade as well and using ACF and custom fields as well as layout builders at the moment Elementor or we could even add WP5 Blocks in that mix? …
Unfortunately Grav doesn’t come with a plugin like that. It might be a limitation of the frameworks possibilities… or maybe someone could build it but it might be a massive task for which revenue? I just think the market is too small for someone to jump it…
Anyhow - you are able with Grav to create custom fields. If you used Wordpress and ACF or you know about the WP custom fields - then Grav comes with a similar counterpart. This allows you to break-out from you standard Markdown editor per page and build your custom fields and layouts really.
Some of theme themes, I am eg. using Quark as a base theme to start out, also come with built in CSS frameworks such as Spectre which come with a ton of features including Grid (similiar to Bootstrap Grids). This can help to build your responsive columns within the standard markdown editor.
Now I know this is far fetched from a layout builder but better than nothing - at least it allows you to manage some aspects of the front-end layout visually…
Hope this might be of some little help.
Sam