I'm not sure what can be offered here OP. You have an IDVA and have had legal advice.
Your IDVA will advise you on safety planning which is very important as, as you already know, leaving the relationship and the first year afterwards are when you are most vulnerable.
Yes, getting extra locks etc sorted out for peace of mind would be helpful I'm sure, as your IDVA will have advised. I would suggest you ask about a secure letterbox so that no flammables can be posted. However, it all depends on what is available through the scheme.
Getting a non molestation order would keep him away and is an arrest arrestable offence. He already has form and, as I'm sure you know, is usually for between 6 and 12 months. You can get an emergency non molestation order from the organisation I already cited above and he does not need to be forewarned of that.
Are you going to do any kind of counselling? Ask your IDVA about what's available in your local area.
Really it's a matter of finishing the relationship and staying away from him. Using all the support you have around you, your IDVA, counselling and support groups etc to stay away and stay strong.
Use the law when necessary, dial 999 if he comes to your place etc Log all events but again, your IDVA is best placed to advise on all that. Each case of domestic abuse is unique to the individual and there is no carte blanche way of dealing with it.