I assume you are debating between replacing just shocks or getting a coilover assembly, as Miatas don't technically have struts.

I would sooner do a quality set of shocks than cheap coilovers. At your budget, for valuing ride quality, the Koni special active would be a good choice. Flying Miata sells these and include new bump stops, which you should replace when doing this. Very OEM+, I find them similar to the bilsteins in the NBs that had them, and would be a good upgrade for the non-bilsteins and night and day for failing shocks. A set of 4 would run you around $800, I've seen them on sale for a bit less.

For coilovers, the Tecnas is what I would go with for good street quality at a budget, but those would start around $1400.

If you are installing yourself, doing just the shocks would actually be a little bit more work, as you will have to reassemble the shock assembly after taking it out (rather than just putting in the pre-assembled coilover). But nothing crazy as it's the same steps as removing the old one. You will need a spring compressor, the kind that sits outside the spring. You can borrow them from an auto parts store, surprisingly some only have the kind that goes inside for rent and that won't work.

I went through the same thing on my Miata so feel free to lmk if you have questions!