Drastically reducing frequencies, increasing fares and splitting services is what will turn passengers away, all three need to be avoided where possible. Reliability issues can always be fixed by adding in extra running time at key parts of the route, you'll often find that the issue isn't actually the length of the route (a 90 minute route is pretty average) but more lack of decent layover at either end and poor running times throughout, you'll have sections where there's too much running time and sections where there's too little, hence why buses may bunch in certain spots.
The problem is that First have reliability issues all over the place because of running times that's already lost so many passengers over the years. I think most people would take an every 15 min service that's reliable over an every 10 min service that's a mess for reliability like the 6 for example. To me, going from every 10 min to 12/15 isn't drastic in terms of cuts and I wouldn't suggest anything more severe than that. If services are split but become reliable it would also be so much easier to schedule services in a way that allowed for good interlinking between services.
Pricing is a whole other mess but it's funding or a lack of it that causes it. Would need to be a 30/35% drop to be affordable for a lot of people now given the effects of the pandemic.
With routes a lot could be altered to be fair. Needs to be a balance of services that cover a lot of places and ones that are direct and get you from A to B much quicker. Areas like Yoker are actually done really well as there's the M11 as a local service, 1s as an express, 2 and 6 that cover the West End and give links to more of the city centre where needed.
A variation of options from one side of the city to the other 3 could certainly give people diversity in where they can go