Glad you enjoy it! Let me know if there are other grappling options you want to see.First time poster here. Thanks for updating this guide, I was thinking about making a grappling build from some time ago, but until I saw all the options and uses It had in combat I wasn't convinced it would be worthwhile, keep up the good work.
Disagree here because you don't always have to Frenzy. You'd just use the ability in your main fight of the day (adventures typically have one of those). You're only going 6 levels deep in Barbarian to get the best of Frenzy anyway, and level 5 is giving you Extra Attack which is usable outside of Frenzy. It's definitely not a gold or sky blue ability, but it's solidly blue.Frenzy shouldn't be rated nearly that high, especially for Grapplers. No-one wants exhaustion, limiting Frezny to 1/day, but for Grapplers every level of exhaustion is a negative for them. Either they're losing speed, or getting disadvantage on their grapple/shove checks, or they're dead.
While prone, the greatsword fighter will have disadvantage on all his attack rolls. You can also start attempting Disarming Attacks once they are stuck on the ground, either through Fighter maneuvers or through the DMG Disarm rule. The DMG Disarm rule is particularly nasty here because they will have disadvantage on the opposed attack roll to avoid being disarmed.Grappling has some cool specific uses but it isn't all that. There is nothing preventing a great sword fighter that you have grappled from chopping you to bits will all of his attacks every round until you let go or die.
Knocking someone prone is nice but YOU are prone too unless you let go. No- you cannot maintain a grapple while standing AND hold a foe prone with one hand-that is stupid.
As for your second point about being prone, that's a purely RAI ruling and you are welcome to make it as a DM (although your player would justifiably argue against it). RAW you are not prone: there is nothing in the "prone", "grappled", "shove", "grapple", or other rule sections that even remotely suggests it.
Moreover, it isn't even a reasonable RAI ruling because there are countless examples of grappling techniques where an opponent ends up prone and you are still standing. I'm normally on board with RAI rulings when they make sense, but this one doesn't. Just look up any instance of "knee on belly" in jujitsu, or the immediate aftermath of any Judo throw.