The Nationwide regular saver was 8% but it’s now only 6.5% for new customers. Also neither issue is strictly speaking “fixed rate”, the APR is variable and they could lower it. But it is fixed term, ie it gets downgraded to a normal savings account after 12 months - so it’s unlikely the APR will be messed with. And as you mention it is a “fixed monthly deposit amount”.
The First Direct account is fixed rate 7% for the fixed term. However it is less flexible than the Nationwide, it does not allow withdrawals. If you need your money you have to close the account and the interest paid is downgraded to the standard savings rate that FD offer which is awful.
The NatWest account is 6.17% APR, also variable, and it’s not fixed in any way really. It is a continuous account, there is no “maturity date”, it lasts until NatWest decide they want to bin it, of course they will probably just lower the rate. Technically speaking you can also put in more than £150/month by using the Round Ups on the debit card, but it’s not worth the time taken to do that on purpose.
So there are catches. Granted I would say the Nationwide and NatWest regular savers have basically no material catches for anyone who is only saving less than £150/£200 a month. Of course it is possible to have accounts with all of them (including RBS which works same as the NatWest), I do that, I have 8 regular savers and there are some people on the MSE forum with even more than me. But I can’t deny it is a bit of a faff to manage, of course I can work around some of the legwork with standing orders but I do have to make sure I have enough in my current account for them to go out at the start of the month.
There is definitely value in a one roof solution for most people who do not want to put as much faff in as you or I. And the truth is unless you have a small amount (Santander Edge, Barclays Rainy Day) the top paying savings accounts are all with small savings-focused banks, I’m not aware of anywhere where you can see your savings balance and current balance on the same screen (assuming it’s not with a savings provider that does open banking) that pays 5%+ that’s currently available to new customers. (Other than Ulster Bank, who I wouldn’t recommend for anyone outside NI as there is admin faff in opening one, and it also has a minimum balance of £5k which is too high for many anyway).
So in that context, even 4.1% from Monzo’s free is not half bad compared to Big Bank savings rates, 4.6% is pretty okay. It’s better than Starling spaces and TSB pots. Going down the Moneyfacts list of easy access savings accounts, the best I can see with a current account is Cynergy Bank who do 4.95% - although I’ve heard bad things about their customer service and they have no app, I’m not sure the current and savings actually are well connected (frankly never even knew they did a current account). Next lowest after that - Monzo!