Sadly OP you are experiencing the rough end of a divorce. This is why many people never want to get married again (no matter what advantages others may advocate). You have also found out that one of the biggest levers in a divorce is the ability to procrastinate. This usually falls to somebody who stays in the house, has a high income and has no reason or need to speed things along. ☹️
If I understand your situation properly, you and your ex have agreed a settlement. He is offering to fund his contribution to you by selling the house. He has put the house on the market and received an offer with a completion date of April.
Your issue is that you know he has a history of procrastination, throughout the divorce and historically in relation to the home. Your problem is that you need to prove this pattern and specifically in relation to the divorce.
Because his offer is an entirely reasonable and very usual one to an outside observer. Unless you demonstrate otherwise. Even then, the court may decide that he should be given the opportunity to deliver on his offer.
I think this is why your solicitor is advising you to sign. He or she doesn’t think you have a realistic chance of winning a case that forces him to release the money by other means. They want you to give your ex the rope to hang himself. To effectively show bad faith. That’s something that takes time, not necessarily money or courts. For now.
The best response is to show you have the time, not always possible I know. Maybe just take comfort that house prices are rising and you may get more money in the end. In fact tell him that you would be more than happy to get a larger settlement in the long run.