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" ... a 6 core i5-10500 cpu, 16gig ram, and 256gig NVME ... " - for me that sounds pretty serious gear for opnsense. Already paid for hardware vs higher wattage usage with mini pc you should consider the years the new fancy mini pc has to last in working condition before it's a breakeven just for the electricity bill. Of course you can try to sell your current in pieces or a whole package - it might get sold easily.
I used an old AMD Kabini 5350 with 60GB ocz for 7-8 years - they were still going on when they got changed to Ryzen 5600 with mirroring zfs ssd:s and yes surprisingly(?) with even higher wattage being used than an mini pc would. If I wanted/needed lower wattage I would consider Lenovos m920x tiny. The 'x' gives 2 nvme slots. But I'll be satisfied with more modern 4 ports intel nic than the current 82571.
I used an old AMD Kabini 5350 with 60GB ocz for 7-8 years - they were still going on when they got changed to Ryzen 5600 with mirroring zfs ssd:s and yes surprisingly(?) with even higher wattage being used than an mini pc would. If I wanted/needed lower wattage I would consider Lenovos m920x tiny. The 'x' gives 2 nvme slots. But I'll be satisfied with more modern 4 ports intel nic than the current 82571.
