As a general rule, Glasgow, the west coast and the islands are a bit warmer and wetter than Edinburgh. I had a lot of wet weather over there, but no significant heat in July 2019. My room in Edinburgh got quite warm one night; it was probably warmer in Glasgow at the same time. My Edinburgh lodging had no air conditioning, and I wished I had it for that one night, but it wasn't bad at all, and I don't recall feeling hot outdoors. I'd probably be willing to risk a place without a/c again if the savings were substantial and I had no one sharing the hotel cost.
I've just scrolled through the historical weather charts for the last 5 Junes in Edinburgh, and I only saw two days when the high went over 75; it got to 79 once in each of two years, so I'd consider that extremely acceptable from a heat perspective. The city averages 10 days with rain in the month of June; there's likely to be more sun in May.
Edinburgh is a very touristy place even outside of August; it's especially noticeable along the Royal Mile and at the Castle.