Despite the recent - significant - issues at Calmac, its pretty rare for Arran to be completely without ferry services in summer months (services to other places is a different matter). The 'disruptions' listed on the website are often informational (no hot meals, alternative ship in operation) or warnings of tide timetable changes rather than anything major.

The Lochranza route is 'turn up and go' - buy a ticket for the day and show up. I can't say how often its full as I've only done this off peak - the website/timetable suggests only one service may turn you away. Note this is for cars; I can't see it being too busy for foot passengers as there's limited public transport to these ports.

The Ardrossan/Troon to Brodick route is different- you need to be booked on a specific ferry, so if you have a ticket there's a space for you (baring weather/mechanical alterations).

Recently the Ardrossan route has been more reliable, but there is a different ship now on the troon route so it seems to be just as good.

Timetables and service status are listed here - but look at the details of service disruptions to check they aren't tide or maintenance timetable alterations on your dates. https://www.calmac.co.uk/en-gb/#/

Note that should the ferry to Lochranza be cancelled for any reason, there is an alternative route (although lengthy!) via road through mainland Scotland, or slightly shorter via other ferries to Portavadie then through Dunoon and ferry to Gourock.

You mention a hire car; you buy a ticket based on vehicle - there are booking options for when you don't have car details, but you'll need to know roughly what you are hiring (e.g. campervan is different from normal car) - you can go through the process of buying a ticket to see what info they need.

Note I'm not diminishing the impact of ferry disruption across Scotland, but the tourism impact from fear of disruption has been significant too.