At my wits end with potty training, 8 months old and still peeing on the bed

I am at my wits' end with potty training my 8-month-old cocker spaniel puppy (girl). I feel like my whole day revolves around thinking about her potty schedule and watching her like a hawk.

We have her on a very strict schedule and yet she still can't seem to hold it more than 3-4 hours, so we take her out to pee every 3-3.5 hours. Still, she keeps having accidents a few times a week and worst of all pees on our bed occasionally (literally right in front of us too!!!!). Almost all of the accidents happen in the evening before bed, so we take her out even more frequently and I've reduced her water intake before bed which has helped, but not completely. She can hold in 8-10 hours at night though no problem.

I have tried positive reinforcement, not cleaning up the accidents in front of her, I give her so much praise and several treats immediately each time she goes outside, but I still don't know why she's not grasping it. I always use an enzyme cleaner and wash the sheets/bedding at 40-60 Celsius.

I don't believe in crate training (yes I know it may be helpful but we already live in a small apartment), but have thought about trying a small puppy pen, I just don't want to invest in something that is only going to be necessary for a few months. Yes, I know we shouldn't allow her on the bed, and we're really working on that.

For info, we live in a small second-floor apartment in a big city, so that also might be having an impact on the process.