Little Warden is a little pricey for me for only 20 URLs. I did find a completely free service yesterday that covers a huge amount of TLDs:
https://dnmin.com/supported-tlds
The only thing that worries me about that one is whether anyone running it can see/snipe my domains. I've sent an email to them asking what guarantees/measures they've got for this but haven't heard back yet. I wouldn't even mind paying a reasonable amount just to guarantee this doesn't happen, but at the same time it's not worth the ยฃ25 a month to me that Little Warden charge. I've used Pinger Man in the past but can't remember whether their DNS monitors cover availability, I'll have to contact them to find out as well as which TLDs they support and the privacy issues. Their prices definitely seem reasonable, since it seems you can pay based on how many monitors you want - $5 a month for 50 is not bad at all.
This is my concern also. Anything that freely gives others your ideas or your data is to be avoided.
I am not sure if domain sniffing still goes on but given the colossal intricacies and complexity of software (plus the software that nobody really knows about), I wouldn't be surprised if this still goes on. Best to assume it does.
It is why I avoid all cloud services and all backup services and alleged domain appraisals (which are largely laughable). The people who run these services and their staff (who are all completely unknown to you) can do whatever they want with your data and you wouldn't know if/when any data was stolen/used/sold, or if you find out some of your data being used, identifying who was the original criminal would be impossible as well as futile. You can't put the toothpaste back in the tube.
In the past I have created email addresses which I subsequently did not use, at all. Not a single time.
Yet some time later, I would receive emails at those addresses.
I used to keep reminders for domains that I was interested in, in case they expired and became re-available, but as you can probably already realise, the good ones rarely are re-available and I am not at all interested in fighting in the murky, grubby and sometimes/often criminal world of domain auctions.
But auctions can yield good finds, because like anything else with humans, there is no pure black and white scenario where everything runs perfectly/error free and so some good names get missed by others, like the one you nabbed.
It just doesn't appeal to me to work that hard.