First, it is very true that the old Home and original routers are challenged with the current level of firmware. It is not marketing to get people to buy new equipment. It has to do with how much code there is, and the processor performance available. It is simply a fact of life if you want continual improvements and features.
To answer the original question.....I'll not repeat what has been said, but simply answer the question I pose in the presentations I do on this topic....The question is "This is all so complicated...What do I do?" And the answer (my answer) is:
Put your money into the best cellular solution you can FIRST. Only then should you do more than the basics of enhancing wifi signal.
That is not to say that the wifi signals are not useful in some instances - they are, and the equipment discussed here will help a lot. But mainly you will be on cellular.
Now I'll wait to hear about the person that has been using JUST wifi for years.....(yes, they are out there, but they will have had limited success travelling, and certainly not for business.)