I expect a fair few PAYG mobiles are like mine, which was last topped up in February 2019...
Not sure when I last put credit on my "bike" phone, last time I used it was to tell Megajoulesexpenditure that I'd bailed out of her 600 2 Springs ago.
It was on around 4 quid then, and it's still on around 4 quid now, so I possibly last topped it up the middle of last decade...
As for charging, possibly more recently.
On top of Harris.
<Andy Peters> Ooerrr Missus </Andy Peters>
I still have PAYG. Cheap Nokia on a different network (vodafone) to my contract smart phone (Tesco/O2) - After 20+ years on Lewis I'm paranoid about not having a signal when in a remote area. The Nokia also has >1 week battery life so is great when on holiday or tour and don't get chance to charge the smart phone.
Vodafone and O2 are mostly sharing networks. So you won't gain much coverage by using one of each.
A separate SIM on EE would be more worthwhile (3 have hardly any coverage anyway).
Historically it was Vodafone that had the best coverage in the isles, to the point certain events required you to have a Vodaphone in order to take part.
Example: My brother being able to phone in that the car he was navigating round mull was upside down in a ditch due to the required Vodafone while my O2 was useless (stupid place for his driver to crash really I had a cracking spot for a photo at Calgary Bay)
Have definitely noticed O2 is much better since the network share began, although there was some Islands where Cellnet got their mast up before Vodafone could claim the spot, Colonsay comes to mind IIRC.