Welcome!
I would add a few more to the above:
Walk Scotland Plus is strong on SouthWest Scotland where you live:
http://www.walkscotland.plus.com/Walking-Routes.co.uk is a website of walking websites:
http://www.walking-routes.co.uk/Frank Firth's excellent site with 700+ curated walks across Yorkshire
https://www.yorkshirewalks.org/Cheviot walks:
http://www.cheviotwalks.org/Further east from where you live, the Scottish Borders Council managed a highly successful project to create walking guides based on each of the Border towns e.g. Kelso, Jedburgh, Galashiels, Newcastleton, etc etc. with a mix of mapped town trails, country walks and hill walks which are signposted on the ground. These are all available on line:
https://www.scotborders.gov.uk/downloads/download/352/paths_around_walks and
https://walkscottishborders.com/ and have GPX files to download.
Although you enquired about on-line resources, I would also make the point that print media shouldn't be overlooked. You might find that a focussed guide to an area to be more helpful. In addition to the Cicerone guides which have already been mentioned, The Pocket Mountains guides are one such range which includes coastal, country and hill-walking routes to some 50-odd areas:
https://pocketmountains.com/Another range which is strong on Scotland is the Pocket Walks series (NB different to the above):
https://www.pocketwalks.com/walking-books/Lastly you shouldn't overlook older out-of-print guidebooks which are available second-hand from the likes of AbeBooks on-line or in used bookshops (think of a visit to Wigtown or a few charity shops!). Authors such as Bob Allen wrote seriously good guidebooks to the Lakes, Snowdonia and the Dales etc which are as good a read today as they were in the 90's. As just one example, his "Escape to the Dales" gives you 45 classic walks, a lively commentary, and sketch maps - a steal at £2.38 inc p&p:
https://www.abebooks.co.uk/servlet/BookDetailsPL?bi=22518588121&searchurl=an%3Dbob%2Ballen%26kn%3Ddales%2B%26sortby%3D17&cm_sp=snippet-_-srp1-_-title1Ken Wilson's seminal trilogy "Classic Walks", "Wild Walks" and "Hard Walks" are coffee table format books and not for taking on the hill, but have stunning images and are certainly still inspirational today. If you are into hill-walking you can probably buy the three for around a tenner, you won't regret it for a moment. Richard Gilbert, Terry Marsh, Ralph Storer are a few other recommended guidebook authors to look out for.
Past editions of Cicerone guides are also readily available and are cheap as chips, giving you all the usual information about the routes which won't really have changed very much, particularly in the hills.
That should keep you going for a decade or two ... it certainly did for me!