Camera upgrade advise

Hi all, I am looking for a bit of advice for upgrading my camera equipment.

So I have had the cannon Eos 4000d for about 6 years now, using it with the standard canon 75-300 lens. I am now ready for an upgrade!! I mainly photograph wildlife, big, small and everything in-between!

I have spent the last 6 months using a friends bridge camera, Nikon coolplix p950 so I got very used to the zoom which I am now missing having gone back to my dslr, I know this is something I won't be able to fix without an expensive lens but, I have also found that going back to my dslr it feels so grainy and just poor quality.
I want to upgrade my camera body firstly so I can practise getting sharp shots even if the zoom isn't very big and one day I want to get a 600m lens just not in the budget yet!

I have been floating around the idea of the 6d or 7d mark II. But I have also heard great things about the 90d and I'm just feeling a little overwhelmed by all the options!!

Budget is going to sit at £600 max, I can get a little back for my 4000d so I will use this towards the new body.

Finally, can anyone give me advise on using a teleconverter with a 300m lens to get that little extra reach? or maybe a different lens that won't break the bank but offer more zoom.

Thank you in advance :)