I would take another thing into account: The former employer are strange people. Strange people mean trouble. You talked to them about backups, repairs and faulty stuff like ups.
They didn't understand about backups and the thought You were threatening. They seem to be crazy people. Crazy people guarantee trouble.
Can You prove that You weren't given permission and financial funds to create backups and make repairs or that they denied (read have signed voucher)? If not, they could always claim they had instructed You to do so and that You had claimed the assignments fulfilled before Your departure. I mean, this wouldn't be the first time I see crazy desparate people lying in court.
So if You're not sure about the way they might react how long their equipment might survive, and if financially viable (I don't know the rates at Your place), I would consult a lawyer (I'm no lawyer, really, so don't consider doing this on Your own, especielly without a lawyer's scrutiny on the matter) and if legally advisable, I would have him notify the company in writing of the following:
as notified several times during Your employment, there were no backups made and there was no sufficient time to do so. Also lack of capable hardware and reliable hardware from a factual and professional point of view and lack of willingness to invest in reliable hardware stymied such attempts.
Backups are no threat but a mandatory best practice in the business because they offer improved protection for the company assets constituted. Many desaster scenarios like virus infection, hardware failure due to ageing, natural forces or the elements for examples, data might and will be completely or partially lost which means that it might not be completely or not at all recoverable from the original locations or devices. Also the cost of backup infrastructure is cheap in comparison to the cost for data recovery, the success of which can't even be guaranteed. Also there is a permanent statistical likelyhood that devices break down due to ageing or other environmental factors which can't be mitigated in the usage of electronics and computers.
This is why, apart from other reasons, their lack of willingness to invest in mandatory repairs, updated or reliable hardware and data safeguarding measures, which were brought up severa ltimes and denied by them, made further employment with them impossible from a professional point of view.
No liability whatsoever can be assumed on Your side whatsoever, given the facts and considering the termination of the employment.
However, as there might not be a successor to Your post, You offer them, for a premium rate, running a one time backup or implementing a backup solution, and are willing to sign a non disclosure agreement if they are willing to do so, too.
However, no liabilities can be assumed after the creation of the backup or the implementation of the solution, especially if no service agreement is made.
They have 14 days to react to this letter or state anything concerning the matter, oterhwise the matter is to be considered concluded after this time.
And whatever the lawyer might want to have added.
If this is applicable legally, wait for the reaction (especially any claims for damages) and delete Backup after a month.
This is not to say that I didn't like the other suggestions. They are good, really. Going the easy way, keeping mum about the backups and deleting them would be my usual bet. But if I have to do with potentially crazy people, I like to confront things because they tend to get back at You at times when it's too late.
Also think about the NDA forms. I'd have terms and conditions requiring customers to do backups and no obligation on Your side to do backups on Your side be assumed except having ordered Backups to be done and received an autographed copy of their backup order from You.
Also, I would't be shy to write my employer a validated letter even during employment confirming his unwillingness to adhere to stated mandatory or advisable best practices and no liability on my side. Also requiring them to have written confirmation for any kind of order to now implement said formerly denied best practices, just to make sure there will be no misunderstandings if they changed their mind.
(Anything where You get a copy of Your letter with a signing it was delivered containing this text should do.) This might be a bit picky but can save a lot of hassle.
But never forget: this is not the way it usually works, as employers most often understand, given the proper explanation. I never had to go that way.
Edit: It just appeared to me that I skipped half of the idea about when to delete the data:
If this is applicable legally, wait for the reaction (especially any claims for damages) and delete Backup after a month.
By that I meant that if there is a legal option of setting a deadline for claims to be made, You can keep the data as some kind of insurance for worst case (complete server self destriction on Your first day off) . If no claims surface until that deadline (plus a considerable safety margin) You can delete the data as nothing can happen to You any more.