Your 10GB plan probably won't work, I think it uses only RAM. But it's unlikely that you'd ever need that much anyway, records time out, they don't last forever. But it of course depends on how big network you have.

Other than that, all resolvers are same in principle. If you need only bare basics, no extra features, it's there. I'd be a little worried about performance, but I haven't seen any benchmark. Maybe if you increase default limits for concurrent queries, it could handle it, I don't know. But default values are not very optimistic. It would be interesting to test what it's capable of.

One more thing that could possibly cause problems is the fact that unlike other resolvers, MikroTik's does not keep letter case used in queries and sends lowercase names in responses. It should not matter, but it seems that maybe in some cases it can (there were threads about Playstation problems suggesting that this may be the reason, but no definitive proof).

But if I was ISP, I'd want some extra features like own DNSSEC validation, and that's something that current RouterOS can't offer.

New Requests/sec: 1.300
Total Requests/sec: 7.322
DNS Records in Cache: 16,079

I can't tell how much will 4k devices need, it depends on what they do and it can vary greatly. The same goes for concurrency limits. It's not like normal device sends dns queries all the time, so average rate should not be high. You should have some space for spikes, again the same thing.

I'm big fan of trial & error method, so I'd start with smaller segment of network and see what it does there. Or before you jump to that, you can add passthrough rule for port 53 and see what's current packet rate. With default udp, it's one packet per query, so it's easy to count. And tcp should be used much less than udp.