Ask anybody running a paid channel where their next hundred subscribers are coming from and they will talk about traffic. Ask where the last hundred went and they usually do not know.
That is the wrong way round. Somebody who has already paid you once has already made every hard decision: they trust you, they know what they are buying, and their card is on file. Keeping them costs a message. Replacing them costs whatever you pay for attention.
Most churn is not a decision
A large share of cancellations are not cancellations at all. A card expired. A bank declined a renewal because the charge looked unusual. Somebody meant to resubscribe after a quiet month and then forgot, because nothing reminded them.
None of those people chose to leave. They just stopped paying, which looks identical in your figures and is completely different in what you should do about it.
The three messages worth sending
Before it ends
A few days out, say when it ends and what happens. Not a sales pitch — a fact. People who intend to stay are grateful, and the ones who meant to cancel get to do it without feeling tricked, which is worth more than the month you would have squeezed out of them.
When a payment fails
Providers retry a declined card for days. The subscriber usually has no idea any of this is happening. One message saying the card was declined and access continues meanwhile turns most of those back into paying subscribers.
A while after they have gone
Not the next day. A month or two later, when whatever made them leave has faded and you have something new to point at.
Send it where they are
An email about a Telegram channel has to survive a spam filter, an inbox and a promotions tab. A message in the channel's own bot arrives in the same app the subscription is FOR, from an account they already trust.
Use email for receipts and anything that needs to be found again months later. Use the bot for anything with a deadline on it.
Everything above is worth setting up once and never thinking about again. The
version you do by hand is the version that stops the week you get busy.