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The highest level you can go while still having great summons/invasion time is 144. At level 144 you can still invade people who are levels 120-166. Here is the summon/invasion range calculator.
So at that lvl are invasions with red eye orb frequent? And how about the Aldrich covenant?
120 you'll have constant invasions/summons still
SL 140 best meta. Can actually make a variety of builds without being op or squishy. And min/max shines best but jack of trades becomes viable. Highly recommended
How active is the SL 160 meta?

I just finished my Quality Pyro build I made, as well as Ng+3 so I figured that this is my opportunity to get into some pvp action.
Fromsoft should really just group everyone together who has already achieved the soft cap of every stat. At that point even a level 700 would not have that many more stats than a level 160-200 who has already got to 40-50 on everything.
PvP is a lot more fun for me if I'm somehow getting something from it, so getting to a soul level and having to purposely stop leveling up is not as fun.
What is the meta now in 2021? Are folks still stopping at SL120, or has everyone moved on to SL150-200 now?
Soul Level 120/125 is standard for Arena and Fight Club play, but you still get some good co-op play at 140/150. For invasions in early areas, try 40. For mid and very late game, 60 or sometimes 80 is fine. FromSoft eventually adapted the summon range for Blades of the Darkmoon and Blue Sentinels to be broader than at release, so 80 is good and 120/125 also works (to the degree that those auto-summon covenants function at all with the game less populated).

[I haven't experimented with Arena matches at 80 but given that the Ashes of Ariandel DLC is scaled in enemy damage and health for later game character builds, Soul Level 80 could very well be viable, given that this DLC is where you acquire the Champion's Bones to burn and access the Arena.]
The soul level in 2021 is actually 155 sadly. So many people switched to 135 or 140 after struggling to compete with 120s. You can check people's character level in achievements.
119 all day