My first question is why change from the FT891? That’s a nice radio and although you may never use it portable at 100 watts out, it might be good to be able to run at a higher power (40 or 50w) for a short time when the bands are difficult.
If it is really a question of weight, then the fact that you can run the FT817/818 range using an internal battery is where the real weight saving is. The G90 won’t save you much weight on the battery as you will still need an external battery - a LiFePo battery is the normal choice. You might want to look at the G90’s little brother, the X5105 if you can still find one (MFJ?), which also runs 5 watts output with an internal battery (It’s Xiegu’s answer to the Elecraft KX3 but in a more basic and less sensitive form, at a lower price). It covers top-band through 6 metres and has a built-in ATU.
By the way, there are as many positive as negative reviews of the G90 (and probably most rigs). People don’t consider their needs well enough before buying the latest rig and are often disappointed that the rig doesn’t do something that it never said it did. So it’s important to really boil down what you need before making the move to a new (or new to you, second-hand) rig.
The FT817/818 is a good rig but an older design and you may be disappointed with its receiver performance when compared to your FT891.
Having 20 watts on a summit rather than 5 watts, does make a difference on SSB (I changed from an FT-817ND to a Xiegu X-108G (the G90 predecessor without the ATU) and while the increase in power does help getting contacts, it brings extra weight with the external battery).
So my advice is
- re-assess what you already have and perhaps look for a smaller, lighter battery and run the FT-891 at 20 watts output when portable.
- Take a look at the Xiegu X5105 rig with its internal battery and ATU.