Hello!
If you've only come off the Pill six months ago, then I wouldn't be at all surprised if your cycle is a bit all over the place - I remember it took at least six months for mine to straighten out. So, my first bit of advice would be, keep monitoring your cycle for several more months.
You said that from using OPKs you were 'ovulating for five days' last month - but since ovulation itself doesn't go on for five days, that suggests you need to add in some more ways of monitoring your cycle to pinpoint it more clearly. I generally think it's a good idea to use at least two different methods to track your cycle, as it gives you more information. If you aren't already temping, it would definitely be a good idea to start, as this can help to pinpoint when you actually ovulated (although it tells you after the event rather than before). Some women also track cervical mucus, or the position of their cervix, or use a salivary ferning kit. Experiment to see what suits you best, but you'll definitely need to pin things down much more than a five day window.
My own checking-for-ovulation regime involves a combo of two different types of OPKs - the clearblue dual hormone ones plus internet dip strips - plus temping, plus vague awareness of my CM. The clearblue tests measure two different hormones so give a bit more of a run-up, whereas the dip strips are so cheap I can do several a day when I'm getting close. What I was able to identify with that testing was not only roughly when I might get positive OPKs, but also how soon after getting them I would actually be likely to ovulate. For me, that is a lot sooner than many online resources suggest - if I waited as long as many sites say before doing an insemination, I'd be way too late. So, this is an exercise in planning when to inseminate, as well as how to recognise when you're getting positive OPKs. Getting the timing right is everything.
My own ovulation isn't terribly reliable in terms of when in my cycle it happens, but I've found a 12-day tank from Cryos to be fine and you'd probably be OK with this, too - especially once you've monitored some more cycles to possibly narrow down the window a bit (and got the Pill completely out of your system).
If you possibly can, consider having a HyCoSy or HSG done to check whether your tubes are clear. I know it's an extra expense, but it would be a great shame to spend all your savings on donor sperm if it later turns out that your eggs had no chance of ever meeting it.
It is possible to get pregnant using Cryos - I say this through personal experience, having achieved one pregnancy in September (which I miscarried in October), and another in November/December (which is around about six weeks along now, fingers crossed). But, it's also really, really easy to spend a lot of money on achieving no result at all (I have done this, too!) so please be realistic with yourself about your chances and recognise that they are very low per cycle. IUI through a clinic is not hugely more expensive than using Cryos at home, and would almost certainly give higher odds of success, so maybe look into this as well and weigh up your options. I always worry for people who are going to need to throw all their financial resources at a very small number of attempts (not that I have infinite money myself!) so please do balance everything up, and definitely keep working on the timing if you decide to go ahead.
Good luck to you, and please feel free to ask if you have any questions!