I got approx. 600 on my first sitting I think, 740 on my second (in a very high scoring cohort however, easier test!) and then 702.5 on my successful attempts.
My advice would be not to apply to GEM with below 650. It's demoralising having rejections; I did it myself and even though you know it's coming, you ultimately still have that "what if..." painfully drained out of you. Honestly, it's not worth it: you will not get in with those scores, it's a frustrating fact. That's aside from the fact, as you mention, that writing a PS etc is quite a toll in itself. The main benefit, which you've already taken from this year, is essentially having a practice UKCAT run. That'll help you next time around
Double check you're not eligible for one of the non-UKCAT courses (e.g. Bristol, Cambridge) just in case you think you might be a good fit. Same with 5-year if you can (in the unlikely event!) afford £40k on tuition. But in terms of UKCAT universities it's game over for the year, and too late for the GAMSAT.
Using 5-year for interviews is not a bad shout if you have the time and money to apply, I suppose. So long as your expectations aren't to get into GEM I don't see the harm. Maybe try and apply to similar schools to those who have similar interview formats for GEM (MMIs would be useful, bit weird to get used to!)
In terms of strengthening your application, that's really your call as I don't know your background. As a general rule, add something. If you haven't done care experience yet, that's the most beneficial thing. If you can't get it, try some form of longer term voluntary work. Add a bit of shadowing if you're a bit light there: they will ask you at interview about life as a doctor in some fashion or another!
To give you an idea of my route, on my first attempt I had mostly personal care experience and academic experience (research etc). Second time around I added voluntary work, but Warwick (the only one where my UKCAT was acceptable) added the requirement for non-personal care. So by third time around I'd undertaken clinical work (at work, and abroad in Uganda for a short while) to help compensate for this.
Good luck next year -- persistence is everything
It's a question of how much you want it and how willing you are to tick boxes! Nothing more.