Lots of the “indoor” shoes are lower end and focused on the Peloton/Soulcycle crowd. They sometimes have improved breathability, and typically have lower end soles (i.e.not full carbon). As you pointed out, they are for 60min. non-competitive rides.
Personally, I’ve dedicated my old pair of outdoor shoes (Sidi Carbon Ergo2) to trainer duty.
The Shimano IC series don’t seem to have great closure options (velcro or single BOAs) IMO. I’d be more inclined to get a mid-range road shoe like the RC5 and put some more holes in the upper (since you don’t have to worry about gravel/rain/etc. on the trainer).
Get some regular road shoes. Maybe something off ebay that are used and cheap. Those indoor cycling shoes aren’t really geared towards what we train to do
This is going to be one of those things that people will give you all sorts of reasons not to do it but I use the Shimano SD5 sandals on my stationary trainer. Apart from being open it’s a plastic foot bed you can get to and just wipe clean.
My wife recently got a pair of Lake CX201 shoes. While not billed explicitly as indoor shoes, they are incredibly airy with basically full mesh toe up through mid foot. On top of that they have a stiff feeling (I’ve not risen them personally) carbon sole and good boa retention system.