An active French SIM card should work in other EU countries like Greece, since they are required to allow roaming between countries.
So if you're saying your French SFR card is active now(?), it should work. But European prepaid SIM cards tend to have time-limited packages - weekly or monthly, something like that. And depending on the country or company, how long they are active and how you can top them off remotely varies.
I have a Dutch Vodafone SIM I bought in 2017 and have used in several European countries now (not the Netherlands yet) - it is good for six months of inactivity before it becomes inactive. I continue to keep it active by sending a text (even from the US) every six months or so. And, I can top it off (add credit) online using my US credit card. But that's the policy of this SIM card. I have no idea how your SFR SIM works. Check out the prepaid SIM wiki below to read up on SFR's policies.
https://prepaid-data-sim-card.fandom.com/wiki/France