I must suggest you think a bit about what you want to accomplish... I "think" 9-out-of-10 eBike owners are looking for "Comfortable Street/Paved-Path Riding' and very few are looking for "Performance Suspension" (if for no ther reason than slamming a 80lb eBike either up a trail or or thrashing it hard on a downhill run is really kindof iffy at best.
When it comes to "Rear Suspension" - my 1st shot when I am looking for "something more from the rear" is to get the eBike up on my Park stand - secure the rear wheel w/rear subframe - pull the shock - and see how thast rear end moves WITOUT the shock - is the vertical movement restricted - without the shock does the rear have lateral movement - do the rear suspension points need to be loosened - lubricated - teflon washers - or alloy bolts replace with 304 (or higher) stainless steel so they hold proper torque when tightened ?
Once I have played with the arse end of my eBike without the shock - then I know what I might be able to achieve with a quality shock. If you have a tight/rough rear end - then a shock can't do much - if the rear end moves smoothly then "maybe" you can get better comfort by changing thins up.
Bike shocks and eBike shocks are "Performance Shocks" - and you can dial in a riding range from a Volkswagen Jetta to a a Nissan Z-Car but you will never be able to duplicate your father's Cadillac Seville.
DMN makes a dang good bike/ebike shock for the money - I got partial to them in my T/T and Thumper moto days be careful and measure each "eye-end" you have now or you can get off track with poor mounting fitment.
1. Tire changes have brought me "comfort" - going to wired bead and low TPI - with a greater range of Fat Tires out there - call that $125 or so
2. Suspension Seat Post - the ones I own are in the $225-$300 ange - frankly the Redshift Sports Shockstop is my fav at the moment - I love their Arclight pedals so much I am a Redshift Believer - the Shockstop Stem- not for me.
3. Seats I probably have a dozen thrown around my shop on top of whats on my eBikes now - no magic bullet - some are more comfortable than others - all of them are under $40 ea for sure
4. Handlebars and Stem that's a real kicker - if your upper body and weight posture is NOT comfortable for you - your arse and back will NEVER be comfortable. For me a adjustable 120mm-130mm adjustable stem (for me I have not needed a stem riser for my ebikes) - and the widest decent bars I can find with a bit of swept back (current fav Tumbleweed Persuader 800mm - lov it = havent tried their 840mm yet). For me - right comfortale riding posture is Priceless !