You'll want some intensity and ideally some intensity under your own control not only group rides.
Really there's no such thing as 'short lactate threshold stuff' with the exception possibly of some specific HIIT work like Tabatas but you won't do those by accident.
To really target L4 (Threshold) work which is key for building sustainable power, for time trialing, for breakaway and chase power, for recovery between frequent hard bursts in races like crits, etc., you want to do sustained efforts at bare minimum of 10 minutes, 12 is better as a minimum length and most folks do L4 work in 15 to 30 minute efforts. They should be continuous with no traffic interruptions, steep descents where you coast or brake or other things that get you off the gas pedal for more than about 30 seconds along the way. Ride them just below your best pace for that duration, you need to finish the efforts and finish the set if you're doing 2 or more but they should be pretty hard and get you breathing deep and steadily and require focus to complete.
I'd definitely add a day of those each week and personally I do two days of those when I'm not racing on the weekends usually a Thursday and Sunday session. Just adding that to your schedule will make a huge difference but there's a bunch of other focused things you can work on in time like VO2 Max and short anaerobic bursts as well as pure sprints. But try introducing a day or two of focused Threshold work in place of one or more of your 'easy' days and you should see results.
Good luck,
-Dave