Gf Re-entering the UK as a tourist

Okay, so this is going to be a complicated post. I met my girlfriend online six years ago, she suffers from mental health problems including anxiety so we only first met in December 2019. I'm from Northern Ireland and she's from the US, I flew to meet her and planned on staying one week, but ended up staying for nearly the entire 90 days the visa waiver allowed me to.

I rely on savings and am continuing my degree in September 2021, before then we're using the time we have to sort our future together. She's been through a lot of traumatic experiences and is also trying to make a better life for herself with me. She came back to the UK with me after my stay in the US and planned on leaving in March, but when coronavirus happened and with the fear of travelling and maybe giving her vulnerable mother at home the virus we changed her flight to May. That flight was cancelled by the airline and because of the even worse situation she applied for the blanket extension offered by the government and stayed until July 31st. This stated it wouldn't impact her record and she wouldn't be considered an overstayer in the future.

My girlfriend has bad anxiety so I help her do the things she's doing, it was always necessary that I bring her home. Spending so much time together makes our bond very close. I did leave with her, we stayed 17 days in Croatia and I entered the US using my ESTA. My stay here expires on the 15th of Nov. We had planned to apply for a marriage visitor visa whilst here so she could return to the UK with me, marry, then leave and apply for a spouse visa after a few months. She is owed about $130,000 on a life insurance policy but unfortunately did not obtain the money in time to apply and meet the requirements. We also had trouble proving her ties to her home country. She owns the house she lives in with her mother, and she has a power of attorney here, these are the reasons we could come up with. Of course it would be simpler to marry in the US where you don't need a visa to do so, then wait some time and apply for a UK spouse visa when she's had her money for six months in her bank account.

The problem now is we were going to use the marriage visitor visa to also ensure she makes it into the UK next time, but now that she doesn't have it that makes coming back more challenging. She was in the UK for 7 months and 21 days through no fault of her own, it was down to covid and she extended her stay legally as stated above. She would only be out of the country for 108 days (3 months 16 days) if she were to try coming back now, which I know would be difficult. Perhaps more difficult with covid. I thought maybe she could enter through Dublin on a 90 day tourist visa waiver and come to Northern Ireland but I don't know if they count her time in the UK just like the UK border do and perhaps deny her as well? One other idea I had was to go to Croatia with her again for a few months until 2021 and perhaps after that length of time she would be granted entry at the UK border or pass through the e-gates?

I haven't fully explained the complexity of the situation so please don't judge us based on what we're doing, we're trying to do everything legally without running into issues at the border. I would really appreciate some solid advice. Thank you.