There are a few aspects of your gutiars setup that make it not ideal to have switch tunings. My peronal instrument (bass) can take many tunings to a point as the strings are more robust, but you end up with a few issues around the action (the angle of the neck relative to the body, which causes the fret buzz), the nut width and possibly the bridge setup. It's not really ideal, and it's less important for me as chords aren't a concern. There's generally a way around whatever I'm trying to do. I play guitar less, but have run into the same problem. 

My guitar strings are alot more temperamental than my bass ones simply because they are thinner and have alot lower tolerence for changes in tuning. I know you ruled out a second guitar, as ultimately that is the best solution, but you are trying to cover an awful lot ground on one guitar. On guitar the only change I'll really do is low E->D as anything beyond that affects the tension and tonality too much. On bass I tend not to, but have been known to go down a semi tone on the lowest string if I really feel I need the extra range. 

I find changing the tuning too much can affect my playing as the patterns on the fretboard shift. If you're happy with moving the tunings to play then I would suggest:

 - I know you ruled it out, but for the large range you're trying to cover a really cheap second guitar would reduce the strain on strings constantly being retuned, provide a potentially better setup for a different tuning and save you time constantly retuning while practising. If you get two you can cover more ground and have strings specifically for higher or lower tunings. A professional musician will have loads obviously, but for £60 you can get a guitar just to practise on, with a more serious instrument if you need it. It won't be best quality, but may save you money long term with the strings you'll be buying constantly by increasing/decreasing tension.

 - Failing that I'd tend towards strings that are lighter than a middle ground. Whilst having strings too loose isn't ideal I would say it's more optimal than trying to increase the tension beyond what they are designed for. You do have to be careful of not damaging your current guitar as the neck won't be that happy with constantly changing tuning.

 - Without knowing the songs you're playing/writing it's hard to say, but possibly look at playing chords elsewhere on the neck and staying in one tuning. If you're covering songs they may sound different so that's your decision to make.