3 Year Old Dachshund Poops and Pees Inside When Alone?

Let me preface this by saying that I absolutely adore my two dogs. Hendrix (who this post is about) is my little soulmate. We are really close, I give him lots of attention and love, but I don't spoil him. I ignore him when he's being naughty and he responds well. He's gotten a lot better over the past year, but Ive reached a wall in house-training him. I hope you guys can help!

Here's the story:

I've had my miniature Dachshund Hendrix since he was around 6 months old. He's now 3 years old.
When we first got him he was really nervous, and would run in insane circles (sometimes while peeing if we weren't home) all the time. We lived in a house with a garden when we got him, and he was outside with our other dog (fully house trained) when we were at work. Eventually we got a doggie door so they could go in and out. He got used to it and didn't really have accidents inside, although he was a pretty bad submissive pee-er for quite a while.

We moved to an apartment with a small garden, and started to walk them to go pee and poop as often as possible, but he refused for the better part of a year. It was hugely frustrating because we'd spend ages outside, then he'd go as soon as we got inside (in the garden). Once our other dog escaped from the garden (we got her back right away though thankfully!) and we decided to keep them inside when we weren't home.

He started doing his business on the rug/floor - never waiting for us to get home even though we'd let him out in the morning and as soon as we got home and again in the evening.

Eventually he started peeing and pooping on walks, which was great, but he still did it at home when we weren't there, and sometimes if we didn't let him out fast enough.

We just moved again - this time to an apartment without a garden. Our schedule has changed so we leave around 7:15am and get back home around 3:30 or 4pm most days. We feed and walk the dogs in the morning, then go for walks as soon as we get home. We feed them their second meal in the evening and go for one or two more walks before bed.

Evey day when we get home in the afternoon he has peed a bunch of times (sometimes up to 6-8 times - I count the puddles) and pooped at least once or twice, but quite a lot each time. We leave wee pads and he does his business there (although of course he moves the pads around and it's still a bit of a mess), but he sometimes poops once there then again in the hall. He doesn't pee anywhere else though, which is good.

I know he's made a lot of progress, but it's driving me nuts because we have to clean pee and poop every single day. It's the same thing if we go out for an hour or two. If no one is home, he just has to pee and poop. I know for a fact that he can hold it because when I'm at home all day he can hold it from morning til afternoon - even later than the time we usually take him (sometimes he holds it from around 8am til 6 or 7pm). He also lets me know when he has to go out. The only issue is him doing it inside when we're not home. He and my other dog have a limited space, not the whole house, they have a Kong each, chews, their beds, and toys - and each other - so I don't know if he's just bored or has separation anxiety, but I really need to sort this problem out.

We have people house/dog sit for us when we go on vacation. The same couple has done it now twice - they love the dogs and the dogs love them, but the accidents drive everyone a bit nuts. We're traveling again in December so I'd love to have him properly house-trained by then if possible so we don't wear out our lovely friends. It's hard to find people who get on with the dogs so well and that we can trust when we're out of town!

Any help/advice would be great.

Additional info:
I haven't crate trained him. I know everyone says it's the best thing to do, but I don't think it'll work on him. If there aren't any pads put out for him he pees in his bed. I'm not completely against the idea, but probably as a last resort. I know he will bark all day, and crates are pretty hard to find for a decent price where I live :/

We've tried restricting their space, or giving them extra space - it doesn't make a difference.

When he poops/pees at home he does a lot more than when we walk him. He'll poop in the morning then twice when we're not home and again in the evening a bit. When we are home all day he only poops twice a day, maybe 3 times, but all in all the quantity is less. I don't get it. He also seems to pee wayyy more when we're not home compared to when we are home all day and take him out.

By ZS