Hardware Filter Modifications for Quansheng UV-K5 Band Extensions?

The Quansheng UV-K5 (product page archive) handheld transceiver has been notable in cheap amateur radios of 2023 by using both a wideband FM chip BK4819 (product page archive). With support of 18-620 and 840-1200 MHz half-duplex with 6.25/12.5/20/25 kHz channel spacing, this puts the chip in the range of five ITU Region 2 amateur bands (10m, 6m, 2m, 1.25m, 70cm, 33 cm) and spitting distance from a sixth (23 cm). Presumably hitting 4m and the rare 8m would be within reason as well. The easily hackable firmware indicates at least nominal receive capability up to 1300 MHz, though!

First, my readily available references:

• https://github.com/ludwich66/Quansheng_UV-K5_Firmware/wiki
• https://github.com/amnemonic/Quansheng_UV-K5_Firmware

Since I'm not particularly interested in setting myself up with Telegram messaging groups, I can only assume I'm behind on the potential innovations, but am also interested in seeing hardware hacking information available openly more than just the firmware files.

My summary of the Measurements page of the ludwich66 wiki from the earlier portion of 2023 is that the receiver functions in spite of the front end filtering across a pretty broad range. On transmit, of course, the internal low-pass filtering makes itself known and is ineffective under 100 MHz with broad harmonic content while being effective to the level of sensitivity of the test equipment beyond 800 MHz. Some output is detectable at the 500 and 610 MHz points, but that doesn't confirm if the upper range of the BK4819 is doing something on transmit.

So, I'm wondering:

• Does substantially reducing the TX/RX filters enable some more nominal sensitivity across the operating range and any (non-over-the-air) output in the 840+ MHz segment of the BK4819?
• Using the 280 MHz VHF/UHF changeover frequency, can the 144 MHz band be retuned to 222 MHz for acceptable amateur performance in Region 2?
• Further, what does retuning either hardware band look like to further achieve over-the-air operation on 50, 902, and 1240 MHz?

With lower cost HTs being readily available for 2m/70cm (144 and 430 MHz) and reasonably available with 1.25m (222 MHz), I'm thinking that the wider splits are the more interesting options. To be honest, mostly to fill out my band access at a lower cost and compact package compared to discontinued commercial HTs and multi-unit transverters. So, the combo I'd like to end up with would be:

Commercial HT	UV-K5 Mod 1	UV-K5 Mod 2
2m	6m	2m (?)
1.25m	33cm	23cm
70cm

How achievable that is? Well, I'd appreciate any input to the above questions and discussions. Pointers toward existing work that I missed would also be very valuable.