Some thoughts:
Do you want a small camera or a larger camera? If you have small lenses, then a small camera might be preferred. If you have larger lenses, then you likely want the E-m1/OM-1 bodies with the deeper handgrip. For me, the 12-200mm is just at the tipping point of what I want to hand hold on a camera like the E-m5 mark III. On the E-m5 mark III, I found getting a fairly cheap external grip allowed me to shoot with the 12-200mm for longer sessions. However, for the larger lenses (like the Panasonic 100-300mm), I still wanted the deeper hand grip.
Are you adverse to using third party batteries? If so, you probably want to skip the E-m1 mark I, E-m5 mark I/II, Pen-F, and E-P5 cameras, as the BLN-1 battery appears to have been discontinued in 2019.
Likewise, the E-m1 mark II/III and E-m1x may be problematical because the BLH-1 battery those cameras use appears to no longer be for sale.
Now for both the BLN-1 and BLH-1 batteries, there are third party clone batteries. I tend to feel that if you go the clone route, you should also get a battery charger made by the same maker.
Speaking of batteries, if you have a stockpile of the BLS-50 batteries that the E-m10 mark I/II used, most of the Pen bodies (other than E-P5 and Pen-F) along with the E-m5 mark III and OM-5 use the BLS-50 batteries. Other bodies would need different batteries.
Do you want to use a viewfinder? I assume you do, since you asked about a OM-D. If not using a viewfinder is ok, it opens up all of the Pen models besides the Pen-F (Pen-F has a viewfinder). If you want a viewfinder, stick with the E-m10, E-m5, E-m1, E-m1x, OM-1, and OM-5 cameras or the Pen-F.
I assume since you were using the E-m10 mark I it isn't an issue, but note that the E-m10 mark I, E-m5 mark I/II, E-m1 mark I/II/II, and E-m1x bodies uses a TFT LCD viewfinder, which has distortions if you look at the viewfinder with polarized sunglasses and you shoot in horizontal orientation.
I need to wear polarized sunglasses when I'm out in daylight to reduce my migraine, so for me, it is a hot button issue.
The distortion in all of the above cameras, except the E-m5 mark II have horizontal bands where one band is clear and the next band is distorted. I can frame shots on these cameras by using single point auto focus and putting the focus point in a clear region. The E-m5 mark II's viewfinder unfortunately becomes opaque if I view it with polarized sunglasses and shoot in horizontal orientation.
The E-m10 mark II/III/IV, E-m5 mark III, OM-5, OM-1 mark I/II and Pen-F all have an OLED viewfinder which does not have the distortions with polarized sunglasses (it may make one orientation darker, but it isn't distorted).
Do you want the body to be splash resistant and you have splash resistant lenses to go with it, then you want to limit yourself to the E-m5, E-m1, E-m1x, OM-1, and OM-5 bodies.
Do you want to record 4K/C4K videos? If so, restrict your choice to the E-m5 mark IV, E-m5 mark III, E-m1 mark II/III, E-m1x, OM-1 mark I/II, or OM-5 bodies. In addition, all of these bodies also take a 3.5mm audio plug to use an external microphone. If you also need to use an external headphone to monitor the sound, then eliminate E-m5 mark III and OM-5.
Do you need to record more than 30 minute clips with video? If so, your choices are the OM-1 mark I/II or OM-5.
If you want your camera to last the longest on a single battery, then your choice in order is:
- OM-1 mark I
- OM-1 mark II
- E-m1 mark II/III and E-m1x
- Most other cameras.
If you have the money for it, I would recommend the E-m5 mark III (or OM-5 if you have more $$). It has the following advantages:
- They have the OLED viewfinder for use with polarized sunglasses.
- They use the BLS-50 batteries that you already own.
- They do 4K video if you prefer.
- They are splash-resistant if used with a splash resistant lens.
- They have phase detect auto focus for better continuous focus.
Now, if you are going to be shooting with larger lenses, you want things like bird-in-flight AI, then you probably should think of the OM-1 mark I instead of the E-m5 mark III/OM-5.
FWIW, I've shot with:
- E-P2
- E-PM2
- E-M5 mark I
- E-M1 mark I
- E-M5 mark III
- E-M1 mark II
- OM-1 mark I
At the moment, the OM-1 is my camera of choice. The E-m5 mark III is my secondary camera. The E-m1 mark II is used in special cases. The E-m1 mark I, E-m5 mark I, E-pm2, and E-p2 are all retired.