Sir, it is truly great to have you back. I am not the only one who were concerned about you when you wasn't here for a while.
Do not give up, there are many of us who go through a living hell due to cov-19 too. There is one thing that you have never lost; Your experiences, passions, knowledge, and anything that makes you being you. It can not be stolen nor lose.
If I ever had a lot of money, I would love to invest into your new combat sim.
However, as others have stated, do not give up. You can start small again and try different various game engine. Unity and Unreal Engine have evolved a lot. Even though there are issues with it, you can eventually make it work out. Plus, Unity and Unreal are not the only engines out there.
Even though if you decide to use an existing sim / game engine instead of building a new one for now, it will be worth of your time, in my opinion. That way you can show the world what you are aiming for. If you eventually to get it working out well, you could start selling the small sim for a low price, in order, to build the financial for a bigger project.
Just thinking of ideas aloud.
P.S. One more thing, If you need or want any help with the GUI designs for menu and screens, I am more than happy to help you with it. Here is an example of my new menu / GUI mod for SF2:
https://combatace.com/forums/topic/95923-eagle114ths-new-sf2-menu-project-wip/