100-400 L II for wildlife photography Strengths and weaknesses

I was planning to get the 100-400 II 4.5/5.6, which should be an L lens and therefore excellent in terms of optics and construction. I also plan to add a 1.4x teleconverter to increase the zoom without having to change lenses, and I've read that with L lenses, given their optical quality, this can be done without too many compromises. I would use it in a Canon r7, so APS-C, and with the 1.4x I would achieve a zoom of 900mm, which I think is fine for all wildlife photography. I'm just afraid that the weight will be excessive (with all the accessories, including the RF adapter, it would weigh almost 2kg), as I would always be holding it freehand. I'm also afraid that starting from 100mm, I won't be able to photograph and record animals up close (perhaps when out on a boat at sea). I would combine this with the RF 18-150 that comes free with the kit. The problem with this set-up is that, since the 100-400 is an EF-S, I have to use the adapter, and if I need the 18-150 to record an animal that is approaching, I have to change everything and then put the adapter back on to put the 100-400 back on. I don't know how inconvenient this could become. I would like to be able to use these two lenses to shoot and take photos of a wide range of wildlife and different environments: sea, forest, mountains, etc. What do you think?