RB960PGS full use of 200/100 Fibre?

Hello

Would the RB960PGS be able to fully use (no bottleneck) a 200up/100down Fibre connection (not using sfp, but Ethernet to ISP ONT) to serve multiple clients with PPPoE/radius accounts, limited accordingly and the bandwidth managed with queues?

Thanks

No.

Would you care to please elaborate and/or suggest a device that would be able to do this?

For a cheap device you can use RB1100AHx4 as it will be OK for about 200 PPPoE Clients but not more (Tested to one of my clients)
For a good device during to 500 PPPoE Clients you can use CCR1009-7G-1C-PC

And of course the RB1100AHx4 does not have SFP so is not suitable for him.
The CCR1009 is a good candidate.
Depending on his actual use he could also consider the RB4011.

I suggested RB1100 AHx4 because he mentioned that: “not using sfp, but Ethernet to ISP ONT”. Yes I’m aware about ONT, if this little device will be capable to take all this traffic all time without problems!
A suggestion for you is to make that ONT as Bridge and config ISP settings to Mikrotik!