Yeah, fuel consumption is not great with the 4-Runner. It seems that the last generation of Toyota off-road vehicles all had bullet-proof power trains, but lousy fuel economy (the 5.7 V8 in the 200-series Land Cruiser, and the 4.0 V6 in the 4-Runner). I refer to the current 4-Runner as "last generation" because (as you said) it is on the way out. In its place is going to come a much more techy version, that will be lighter, and with a smaller forced induction engine. It will be a similar re-vision as the Land Cruiser went through (the 300-seres has more tech, is lighter weight, and has a smaller forced induction engine than the 200-series). The first Toyota off-roader to go through this transition was the Tacoma; the 3rd Gen Tacoma lost its fully-boxed frame in favor of a lighter C-channel, lost the old reliable - if thirsty - 4.0 V6 and got a 3.5 V6 that has no torque below 3200 rpm, and lost a manual transfer case in favor of a dial and an electronic system that is slow to engage 4-lo. In my opinion, the 3rd Gen Tacoma is riding on the reputation of previous generation Tacomas and other excellent Toyota off-road vehicles. I think it will have the shortest production period of any Tacoma, and I expect a 4th Gen soon - probably with a forced induction 4-cylinder engine - similar to the excellent 2.7 Inline 4-cylinder in the 2023 Chevy Colorado (also used in the full-size Chevy Silverado).
Toyota Tacoma naturally aspirated 3.5 V6: 278 hp / 265 pound-feet of torque.
Chevy Colorado turbo-charged 2.7 I4 (high output): 310 hp / 430 pound-feet of torque.
If you want a 4-Runner because you prioritize simplicity, reliability, and relatively "old-school" tech, I'd say buy one now before they are gone. I think the price of the used 4.0 liter 4-Runners will go up just like the price of used 200-series Land Cruisers and FJ-Cruisers here in North America have gone up; a lot of people still want an "old-school" off-road vehicle. A 2014 FJ-Cruiser recently sold for $50,000 not far from where I live. The two major downsides to the current 4-Runner are the 5-speed transmission (way out of date), and the fuel economy. If you are really into off-roading, then the lack of a solid front axle, and the lack of a front locker are also downsides.
The downsides of the 4-Runner are probably what have you (like me) very interested in the Grenadier. The ZF 8-speed seems to be the best automatic transmission available in an off-road vehicle. We don't know how the Grenadier drives on-road, and maybe the IFS of the 4-Runner will handle better on-road than the solid front axle in the Grenadier. If that is the case, then one has to decide which upside to prioritize, and which downside is something one can live with.
As to fuel-economy in the Grenadier, we really don't know yet. I am not overly optimistic about fuel-economy, but I don't think it will be so bad that I won't buy it. Here in North America, real-world fuel economy in forced induction engines is always worse than what the EPA lists for the vehicle. The EPA test does not spend much time with the engine in higher-boost, and so fuel economy looks okay. Most people get into the boost more that the EPA, and see worse fuel economy. This is especially true if you live at elevation, tow a lot, or like "spirited" driving