Re: What cycle computer?
I generally interpret "Cycle computer" as a lower end device that uses a wheel sensor (possible a pedal crank sensor) to measure speed and distance (and possibly cadence) from which it will also calculate average speed, ride time. some include temperature and some sort of altitude sensor.
GPS cycle computers are normally the ones with mapping, turn by turn directions and a variable range of training and other sensors covering a massive range of budgets.
People get to have favourite brands which is normally the brand they use. I use a Garmin, it does more than I want or need and I don't particularly like Garmin as a company but I got it when my Wahoo device let me down really badly middle of a 2 month cycle tour in EU. So from my personal experience I won't touch Wahoo again
https://psamathe.net/wahoo-elemnt-navigation-review/. Others seem to love Wahoo.
I'd set budget first, and post looking for something in range £xxx-£yyy as even eg comparing lower end Garmin devices to higher end Garmin devices gives you very different functionality.
In terms of creating routes to follow people use different onling systems. I use and am well impressed with
https://cycle.travel but there are others. cycle.travel is free and over many months and many thousands of miles touring in Europe has never given me a bad route and always well suited to cycling. When touring each evening I decide where I'll go tomorrow and create a route and on the cycle.travel website I create the route, press a single load to Garmin button and it appears on my GPS cycle computer ready for tomorrow. I have tested some other online cycle route creators and for my test route some have provided a route I can only describe as "certain death" - no way would I cycle along the A14 round Ipswich (3 lane dual carriage way with no cycle lane) - so I stick with cycle.travel which I know works.