I can't see why you need to switch from PAYG to a contract in order to "avail ourselves of the various security devices that work on phone apps". Your phone (and the apps on it) have no way of knowing whether you're using a PAYG tariff or a contract one.
My own suggestion would be to use a 'midway point' between a basic PAYG tariff and a full contract by buying 'bundles' of data and call time that automatically renew each month. i.e just like a contract, a certain amount of money will be taken from your bank account reach month but, unlike a contract, you can increase/decrease that amount (or simply end the arrangement altogether) at any time. You're technically using PAYG but you get the advantages (and none of the disadvantages) of being on a contract.
I use Asda Mobile and currently pay £12 per month. However that gives me far more calls and data than I ever seem to use, so I can probably drop that down to £10 per month, or even possibly to £7 per month. With your minimal needs, you might well be able to pay just £5 per month with Asda Mobile:
https://mobile.asda.com/bundles
Asda Mobile uses the EE network of transmitters, which provides good 4G coverage almost everywhere.