I've run the whole Spelljammer 'Light of Xaryxis' campaign with A5e. No problem at all, but fights can get a bit easier, depending on classes in the party (Arachnid Guardian Adept gets f*cking awesome beyond level 5...). Adding some Exploration Challenges spices up the whole thing.
As I understand and run it., Action, Reaction, Bonus action are the TRIGGER for the respective maneuver. So, you take the Attack action and therefore trigger a certain maneuver which grants additional EFFECTS to the attack.
It's the same as with a Shield-spell using a Reaction, or a Misty Step...
The campaign setting I'm working (and playtesting) on basically uses the rest rules from Ruins of Symbaroum. Breather after a fight for 1d4+CON HP, Short Rest at 4 hours (Hit Dice use as normal), Long Rest at 8 hours (Max of 1 Hit Die in HPs, normal HD use) and Extended Rest for 1d4 days at a...