EOL Enterprise Firewall Questions and Recommendations

Hello. We have a Cyberoam CR 500iNG-XP in our office but it is due for EOL next month so in my understanding the support and the subscription based features will also end. So my first question is, with appliances like this, is there anything else we can do to this after retiring it? Can we repurpose it to a let’s say a linux server or are these things locked? Or is it wiser to look for exchange programs with suppliers? Pardon me if this is a dumb question. Second, in buying NGFW’s, how many years normally is the life of these appliances before it goes EOL? Lastly, I’ve found these recommendations to replace the said model in other forums, mentioning Palo Alto PA-3000, Sonicwall NSA4600, and Fortinet E or F series. I'm still checking them out. Any comments on these models? Thanks!