What I recommend is that you install vanilla RTW and patch it up to 1.2. Then simply copy the entire RTW folder and paste it twice.

With your original folder, install BI and patch it up to 1.6.

With one of the copies, install RTR 6.0 Gold. Eventually 7.0 will be compatible with BI but it's not out yet.

With the other copy, probably wait for the patch for Europa Barbarorum which apparently is imminent.

Then you will have the best of all possible worlds - BI, patched RTW, RTR and EB.

I would definitely try BI before you go back to vanilla RTW or indeed RTR. The strategic aspect is different and interesting (I really like the WREs situation, for example); and I think the combat has been tweaked so that it is a little more like MTW and RTR.

I've been meaning to go back to vanilla RTW with the latest patch - and I enjoyed a 10 year stint as Seleucia in the Throne Room. But I am not sure enough has changed to make it better than the mods.

RTR is my current mod of choice, but that's mainly because the EB beta crashes so much on my computer. EB has an awful lot of potential.