With distances that big, is it even necessary to slow down much? The depth of focus is probably a couple dozen AU? Even if it takes the probe a century to get there, if you can squeeze a decade or two of observation out of it without slowing down, there's no reason to bother and instead send a new upgraded telescope every decade or so.
As far as power requirements go, assuming a doubled power demand from Hubble might be a bit excessive. A telescope that far out would have to be nuclear powered, so thermal regulation is 'free'/passive and RCS load is reduced (don't have to constantly adjust to point away from the Earth), which I expect are the biggest power draws on Hubble.
If we assume a 150 year lifetime, with a 3kW draw by EOL and current RTG tech... RTGs have ~6% efficiency, so for 3kW electricity, you need 50kW in heat. RTG electricity output drops ~2% per year, so after 150 years, you have 5% of the initial electrical output, and you get ~0.57W/g of Pu-238. Meaning, you need ~600kg of it to power the telescope this way [https://www.mathscinotes.com/2012/01/nuclear-battery-math/].
That's not a politically feasible amount, but it's not technically impossible with current/near future tech whose development could be spurred on by serious interest in this kind of mission.
'Proper' fission reactors can also do the job, you get higher efficiency and don't have to run the reactors for the entire 150 years besides accounting for decay (e.g. an RTG that needs to provide enough power to keep some clocks running, the electronics and batteries warm, and trigger whatever mechanism would start up the reactor). Probably less than 100kg of Pu-238 just by better reactor efficiency.