In general, the higher you go up the ladder, the diminishing returns start to kick in hard.
If I could give a ballpark figure, from budget -> midfi -> TOTL IEMs, each bracket gives u about 10 - 30% improvement sonically (subjectively) compared to the preceding bracket, but the price is exponentially more by almost 3 - 10 times each price bracket you jump up.
The sweetspot I would say is somewhere in the $100 - 300ish regions, that's where the most bang for buck is, IMHO.
U can consider the LZ A7 if u want a good upgrade. It has 10 tuning sound signatures, anything from neutralish to U shaped to V shaped, so very versatile. Excellent technicalities, fit, isolation and very good timbre for a hybrid containing piezo. Easy to drive too, no amp needed.
For an amp, it would be useful for certain IEMs with very low sensitivity +/- high impedance, or with certain driver types like planars. Dynamic drivers in general also scale better with a dedicated amp. Multi BA types generally don't benefit from amping, and in fact too much power may skew their frequency response.
IMHO, in terms of price to benefit ratio in the audio chain, I would say upgrade the transducer first (eg the headphone/earbud/IEM), then the amp later on if you have cash. Eartips are also very important in the sonic chain, but its a YMMV situation as we all have different ear anatomies. Thereafter if u still have cash to burn, then u can explore different DACs, Music file bitrate and type (eg lossless vs lossy) and even cables. I would put cables as the last area to upgrade.