You could always buy a ready to go one like this https://www.ebay.co.uk/itm/Roundup-Fast-Action-Weedkiller-Spray-Ready-to-Use-3-L-Garden-Root-Weed-Killer/322524709603?epid=2249455277&hash=item4b17f882e3:g:CPIAAOSwfRdZIkbs
And, of course, there's absolutely wrong with traditional methods...we hoe and hand weed where ever possible However, weeding the rock garden's paths by hand can take several weeks.
I've used Glyphosate in fully stocked ponds before to kill invasive waterlilies and bullrushes with no impact on the environment. I use it sparingly in the Rockery and I have a fantastic array of amphibians, birds, insects and mammals.
I have, however, grown a second head and I have webbed...
Valerian (Centranthus ruber)?
With regards to the original post, Jack Straw has summed it up pretty well (he's learnt everything from me!!!!)
I would suggest you sit there for a few months and decide what you would like. Scent, roses, annual bedding plants, shrubs, herbaceous perennials...