You can use a small Nextcloud in combination with collabora. As far as I know there’s still a free option for a small Nextcloud with up to 8 GB of cloud storage currently. To work on your various documents on the go I recommend collabora, which I use on my phone. On your desktop you can use any number of office applications that support the file standards you’re working with. I haven’t tried collabora on desktop, but it supposedly works there as well. I personally use Softmaker’s Free Office Suite on desktop with cloud storage on the Nextcloud. So far I’ve managed to keep this entire setup free and (apart from Softmaker’s Office) open source.