Tips/advice for leaving older puppy alone

Hi all, I am looking for a bit of advice/reassurance please!

We have a 6-month old puppy (Maple, yorkie x jack russell) who’s hardly ever been left alone. Between me and my partner working from home and taking her out with us most weekends, she’s almost always had company. We’re really lucky to be able to spend so much time with her, but we’re starting to worry we may have set her up for separation anxiety. At the moment we don’t need to leave her, but we’d like her to be comfortable being alone for an hour or two if needed, as things do come up and we sometimes have to scramble to adjust plans.

We’ve recently started leaving her alone for 15-20 minutes and plan to build it up gradually (we started with just a few minutes at first), but she cries and barks a lot when we leave. We’ve even heard her barking from outside when coming home, so I’m worried she might be doing it the whole time. Is that normal at this stage?

We leave her with things like a snuffle mat, puzzle toys, or a frozen Kong, but she doesn’t always seem interested when we’re gone (sometimes treats are left). When we’re working from home she’s quite independent – she’ll sleep, play on her own, and is fine going into other rooms or when doors are closed. She’s also stayed overnight at my mum’s house with no issues, so she is happy being with people other than the two of us.

She isn’t crate trained which I know can be a bit of a mixed topic but she just did not take to the crate at all even after a long time of trying. She loves her bed and happily spends time in it. When we leave her alone, we leave her in the living room which is where her bed and all of her toys are.

Have we left this too late at 6 months? Will the barking settle with practice, or is there something else we should be doing to make this easier for her? I just don’t want her to be so stressed out 🙁

Thanks so much in advance!