You should define "big impact" - what metrics are important to you? Which improvement gives the biggest customer value? You might apply the same measures, that are used to priorize features in the backlog (for example: Backlog Prioritization techniques).

Try to estimate what the impact of these improvements in a long term will be. Are you able to deliver features faster and with better quality?

If there are critical issues / bugs that might break the whole application, they should be fixed as soon as possible!

Dealing with technical dept can be a thin line, where there is sometimes no right or wrong - but there are approaches to do so:

• 11 Strategies for Dealing With Technical Debt
• Escaping the black hole of technical debt

In general the agile way is iterative, maybe you try to split the big problems into smaller ones and improve your process / code base in small steps (Although I know that some refactorings have to be done in one big step).

You should define "big impact" - what metrics are important to you? Which improvement gives the biggest customer value? You might apply the same measures, that are used to priorize features in the backlog (for example: Backlog Prioritization techniques).

Try to estimate what the impact of these improvements in a long term will be. Are you able to deliver features faster and with better quality?

If there are critical issues / bugs that might break the whole application, they should be fixed as soon as possible!

Dealing with technical dept can be a thin line, where there is sometimes no right or wrong - but there are approaches to do so:

• 11 Strategies for Dealing With Technical Debt
• Escaping the black hole of technical debt

In general the agile way is iterative, maybe you try to split the big problems into smaller ones and improve your process / code base in small steps (Although I know that some refactorings have to be done in one big step).