It’s strange that it has a 90 for the clean out and also has an open connection on the right. If this is a vent stack it should be stinking up the place always. I guess maybe when the washing machine drains it creates some negative pressure that causes methane to leak in. If the vertical pipe is actually a vent stack you should be able to plug the open connection on the right side. Another comment is make sure any p trap has a bit of water in it. Traps will also leak methane when they start to dry out.

Where will the target be? Online or local? Rsync is really easy to use and the target files are browse-able. I could be too dense but I find online buckets aren’t easily browse-able. Even a homemade NAS might be a good choice and it’s easily scalable.