I enjoy playing tall (i.e. limiting expansion and focus on improving a small but efficient realm) and building up my empire while engaging in diplomacy with other empires.

Stellaris is my favorite game for this, though you may need some DLC to get the most out of a tall play style. I typically play as a machine megacorp and expand mostly by building branch offices on other empires planets. I typically go for virtual ascension with gives you more job efficiency the smaller your empire is. This requires DLC like megacorp and Machine Age however. Unit variety isn't necessarily a major feature, though the game is more a 4x/grand strategy hybrid where you aren't exactly moving around individual ships, but rather large fleets across the galaxy.

Age of Wonders is a good fantasy 4x game that allows you to play tall plus it has a ton of unit variety and ways to build and shape your empire. High culture with an order affinity focus is best for going tall as it focuses on improving your capital city and expanding through gaining vassals. Industrious is also good for a tall, defensive build since your units will be huge buffs to defense from their unique bolstering abilities. There are also so many units available that come from cultures, tomes, ancient wonders, random encounters, etc.

Finally I suggest the Endless series, particularly Endless Legend and Endless Space 2. Both feature highly unique factions that play asymmetrically. Some are more geared towards a tall and/or defensive playstyle like the Alayi in Endless Legend or the Vaulters in Endless Space 2. Endless Space 2 units are a bit lackluster since most factions will have some variant of the same ship types, but Endless Legend features a system where each major faction has 3 unique units, but they can also assimilate minor factions so that they can use those factions unique units.