As soon as I buy a new fabric riding suit, I take the separate liners out. They do not go back in.
Beneath the jacket and trousers of my riding suit, I use:
(A) In summer / any time of year that I think I’ll definitely be warm enough
A simple Merino T-shirt.
(B) In summer / any time of year that I’m not sure that I’ll definitely be warm enough
I substitute a long sleeved Merino top to for the T-shirt. I’ll also maybe have a thin fleece or EDZ thin top, so I can layer up, if necessary.
C. In winter / any time of year that I know I will not be warm enough
Long sleeved Merino top and Merino leggings, plus my electric heated clothing (see your parallel thread).
Possible variations include
I do have two choices of fabric riding suit. One, the traditional waterproof suit, the other a non-waterproof mesh suit. I also have a leather, two piece, riding suit. When wearing the non-waterproof and / or leather clothing, I will take a waterproof cover along with me, if I think it might rain at some point.
If I think it is going to be very hot, I might well wear a very thin Merino base layer, under my jacket and trousers. This is to act as a wicking layer. I definitely do this if I am wearing my leathers.
I do wear Merino underpants and socks, simply as they can be easily washed and then dried.
I also have a one piece fabric oversuit. This I use only when I take my bike to Battersea for servicing, putting it on over the top of my office suit, shirt and tie. I will also wear it, over the top of conventional casual clothes, if I am going to see someone socially to spend the day out. Or I’ll just go in the car.
I also have one pair of fabric waterproof ‘motorcycling’ jeans, that I might wear from time to time, if I feel like it. I don’t wear them under anything. I do not wear them touring, as they often get splattered by bugs and are difficult to wash / dry when away. By day 10, they are probably a bio-hazard.
In short, I have a varied combination of clothing. Mix-and-match and layering, is the way to go. It cost some cash to assemble the lot but, as it has all been looked after, it has served me well for many years. For example, if I notice that a Merino sock has a small hole in it, I’ll get it darned. This simple action extends its life. I would guess that my thin, very simple, Spidi waterproof jacket is over 30 years old. It still works very well. That I can, if I want to, also wear it over my mesh jacket on a cool spring or autumn’s morning or evening ride from home to the Chunnel, is of course a bonus.
In other words, don’t be prescriptive, be flexible. Find what combination (or combinations *) works for YOU and stick to it.
* Pun intended.