Few steps back... (My first response was on my phone and since a lot of people don't respond after a question, I didn't pay much attention)
Where did you read that the SP5 Advanced LTE will not be supported anymore after December 2021? Is there a chance that it is just your unit that is out of warranty then?
Assumuning it is Microsoft stating in general for all SP5 Advanced LTE owners: we will no longer support this device after December this year:
1. Yes, you can definitely still use the product! People are still using the SP1, 2, 3, 4 without any issues!
2. Nothing will change in every day use
3. Windows Pro came with the device, but is separately supported. Make sure you update on a regular basis to the at least the supported version (I always updat to the most current version as soon as possible) and you're good!
4. Well, I wouldn't see in what scenario it would be UNSAFE to use it.
With regards to your follow up question: if you go to your Microsoft account you can check for how long it will be repaired under warranty. I am not sure what will happen if you request a repaid in January 2022. I would think it is not an issue as the SP6, SP7 and SP7+ are basically still the same.
On the other hand "repair" in Microsoft terms almost always means: replace with a refurbished device. So perhaps the replacement devices are running out. And it would be weird if you send in a device for repair and you get a SP7+ (since that is the only one with built in LTE) Otherwise I will be happy to send in mine for some repairs!