I come from wordpress originally (it was actually bb/cafelog way back then). You are in the big leagues now.
I highly recommend 7 for many reasons. Since my short life with Laravel its the best upgrade. Check out coders tape on "whats new in laravel 7" or bitfumes has a great series on youtube getting started with Laravel 7.
BUT the guts of it will be in 6.
Small things - Personally I love the new casts, stubs, and most of all the http client which is a huge part of my application.
Big things -
UI / Auth:
Laravel feels to me they are shaping Laravel up to be heavy JS front end structure and php backend for the api. This update contains several things for that including their own JS authentication and components as well as more UI scaffolding for js frameworks. Also might as well just get used to returning everything in json (new helper for that).
if that is your thing this contains their new airlock/(whatever they call it) javascript authentication for your apis. Personally I prefer just easy made md5 sum tokens but I guess depends on what you are using it for and how much security you need.
Components:
This is part UI but Laravel seems to be moving so much JS friendly front end I dunno how much the view stuff will matter as pretty much most people return everything as JSON and with all the frameworks out there its so easy... I had never written javascript before but purchased a VUE (vuexy to be specific but there are tons) that already had laravel components... super simple. I guess this is not 7 specific... but the components are.
The helpers!!! So underated. Just look at all those beautiful time saving menthod/functions under Arr and Str.
Telescope, Horizon, Nova updates!!! I use queues VERY heavy with redis and the horizon and telescope improvements are great.
And many more. TBH I see no reason to ever use LTS. Laravel moves SO FAST. Just make sure you only composer update on your dev boxes and ONLY composer install your production =).
I have a really good friend that has a large Laravel site and he is stuck on LTS because whenever he tries to upgrade things break even following the guides. You do not want to fall behind in Laravel. With 2 major releases a year and some pretty big in between... It just goes so fast.
I am trying to come up with a reason you should do 6 and I just can't. I learned with 5.7 vidoes right before 6 came out and 7 out already! The "core" will always be pretty close and upgrading major versions to those core files isn't hard at all. Usually there are many Youtube walkthroughs for it also.
THE ONLY THING is if you use 3rd party packages. I write all of my stuff /helpers and so I fully know what to change if anything. Since you come from the plugin wordpress world you probably already get this and a much better coder than I so should be no issues.
In closing the biggest thing and something I have come to love is TDD (check vids). Constantly run your test suites to make sure everything works. Always test upgrades/updates on your dev box first. This is kind of a DUH. Also always make sure you can quick rollback if needed. Ya I know kind of duh's.
Welcome to Laravel homie!