Would this (company scheme with company matched contribution) be considered the fastest and most optimal way to build up your pension fund?
This is a bit of a red herring. While it obviously helps, it is more important for you to reach the maximum level of contribution that you can afford. At 50, you can contribute up to 30% of your income so you need to strive for this. The best way of doing this is for your wife to return to work as soon as is reasonable.
With the level of debt that you have (mortgage & home loan) and 3rd level on the horizon for the 3 older kids, you are likely to need additional loans when all 3 could be in university together. With a single income, you are at serious risk of missing payments and affecting your credit report which could make it very difficult to access loans in the future
Life insurance:
Yes, cover for 400k+
You are massively under-insured. If anything happened to you, the €400k covers your debts. Your spouse would have no ability to maintain your current lifestyle even if they returned to work. The home would have to be sold to free up money. Ideally this wouldn't have to be done at least until the older 3 are through university and independent
Our mortgage is 3.15 times my gross salary, so it's bit under the new Central Bank lending loan-to-income limit.
IMO, this is a failure of the CB rules. You might have been within the limits but in your mid-40's with a single income, 5 dependents and no pensions, the bank should never have allowed you to borrow that amount as it is now putting you under pressure
there are lots of 3rd level colleges to choose from when the kids reach that age, plus they could live at home if the end up going to college in the area
This is not a choice but a necessity. As your kids approach 15/16, you should talk to them as adults and explain some of the financial choices you are making. You can't afford to pay their rent elsewhere so they have to choose a university closer to home. Encouraging them to get weekend and holiday work will also take the pressure off you
We manage our family budget in a large spreadsheet, every penny is accounted for from the moment I get paid each month. We change gas/electricity/broadband providers every year on the exact day the contract is up to keep costs are a bare minimum. Shop at discount supermarkets such as Lidl to keep costs down. Two cars in the household, both over 10 years old. Nothing fancy, no frills, no crazy expensive purchases, food is always on the table and bills/loans always paid, one "budget" holiday once a year... but yes, it is tight, especially with the cost of living these days.
This is encouraging that you have good control of your budget. You should be able to see how important it will be for your spouse to return to work and the impact that an additional €20/30/40k will have on your ability to fund pension and clear debt.
Overall, I don't think you should sell based on your ideal location for 3rd level (no rents) and the additional needs of one of your children. Its far more important for your spouse to have some form of income.
But even if you manage to get a decent pension fund together, downsizing is always an option when you your kids are independent which will further improve your quality of life in retirement.
You have already made a lot of sacrifices to raise your family so don't start getting any strange notions (like many do) about keeping this home as the kids inheritance. There is a good chance that one of you could live to 90+ so your kids will be in their 50's before that is on the cards. Its much more important that you have a decent quality of life in retirement and selling your PPR should be part of that plan
