I use the RF 24-105 2.8 and 100-300 on 2 R52's for my kid's baseball/softball. I find this combo excellent with 24-300 at 2.8. I agree that 300mm can be short for outfield shots if I am behind home plate, but I move down the foul line if I want to get closer shots out there.
If you do not like the reach of the 100-300 even with extenders, why don't you use the 200-800? I used to use the 100-500 before I got my 100-300. I loved the range and reach, but I really hated the lack of background separation. Unfortunately, everything is a compromise, and unless I am willing to run 3 camera/lens combinations something is always going to be missing/lacking.
I'm strongly considering adding the 24-105 f/2.8L. How often do you find yourself using it during baseball games? My hesitation is I already own the 28-70 f/2, and I'm not sure the 30mm warrants $3k for me.
My reasoning behind the 100-300 was not baseball exclusively. For wrestling, I wanted something that was fast (indoor high school gyms, mostly) but with a decent zoom range. I have the EF 70-200 f/2.8 II, but 200mm simply wasn't enough for a lot of shots. The 100-300 has been absolutely splendid for wrestling. I can get my shutter speed as high as 1/500 and ISO under 6400 in most gyms. The majority of my shots are between 130mm and 250mm, but I enjoy having the options to zoom in for closeups or out for wider shots of the whole mat.
For baseball, I have my first real experience with this 100-300 this evening. My son has his first outdoor game since I added the lens. So far, my baseball shooting has been indoors (horrible lighting) or during outdoor practice where I can walk onto the field. For the indoor tournaments, 300 is enough to get the whole batter in the frame if I'm behind the dugout, but getting players in the field (especially the outfield) is a different story. I'll bring both the 1.4x and 2x TCs tonight. I did a couple rounds of photographing my kids skiing with the 100-300 + 1.4x/2x TCs and I was pleased with the IQ.
My dream lens setup for baseball is the RF 24-105 f/2.8, RF 135 f/1.8 (I have the EF f/2 now, which still holds up remarkably well), the RF 100-300 f/2.8, and the RF 600 f/4 (rumored II). (1.4x TCs in use for the last two)
My reasoning against the 200-800 or even the 100-500 is I live in the northern US (Michigan), so half of my sports photography is indoors, and those lenses are simply too slow for shooting in field houses or gymnasiums.
But I'm always open to feedback or suggestions!