heading back and staying in Selfoss
Selfoss is a working town. You may prefer to stay almost anywhere else.
Also, all the tours we are finding start in Reykjavik.
That's because it's Still Winter and the vast majority of the winter tourists are in Reykjavik. April is Still Winter in Iceland. Of course you can arrange to meet the tours on the way out of R if you are somewhere on the way. There are some local winter tours in the Skaftafell area, though doing anything like going back west to L..r is unlikely, though I realise they do that in summer. I don't know if you'll be too late for ice cave hunting tours that run around Skaftafell, they rely on travelling on winter-frozen lakes so they can't run as late as some winter activities, maybe at that time in the season they can't even say in advance until they see what is happening.
You are proposing quite a lot of driving (especially bearing in mind you are thinking of whole-day tours on top like going to Landmannalaugar), which would be fair enough in the summer, but in winter you need to be a bit more modest in your ambitions, or at least keep a weather eye on the risks. It's Still Winter and heavy snowfall is still fairly frequent, though, as in midwinter, milder wet weather is also possible. You need to keep a bit of time in hand to get back to Reykjavik in case a heavy snowfall delays you a day or something. Of course you may get there and find the roads perfectly clear and the forecast quite clear there is no risk of anything nasty in the foreseeable. Keep an eye on http://www.vegagerdin.is/english and http://en.vedur.is/
